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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallA “Trojan virus” is the common name people use for Trojan horse malware: software that disguises itself as legitimate or useful so someone will install it. Unlike a computer virus or worm, a Trojan does not spread on its own. Once installed, it may steal information, spy on activity, install other malware, enable fraud, or give an attacker access—but those are possible behaviors, not a checklist every Trojan follows.
What is a Trojan virus?
Technically, a Trojan is a category of malware, not a virus. Microsoft describes Trojans as malware that “can’t spread on their own”: they generally need a person to download and run them, or another malware program to deliver and install them. The name refers to the disguise—an app or file presents itself as harmless or useful while concealing malicious behavior. Microsoft’s Trojan overview and Microsoft’s consumer explanation describe this distinction.
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People may use “Trojan virus” to mean any deceptive malware, but self-replication is not what defines a Trojan. A Trojan can install or carry a virus or other malware; that does not make the Trojan itself self-spreading.
Trojan, virus, and worm: what is different?
| Type | Defining behavior | How it reaches a device |
|---|---|---|
| Trojan | Disguises malicious software or activity as something legitimate; may perform different harmful actions after installation. | Typically installed after a user runs it, or delivered by another malware program. |
| Virus | Malicious code associated with or attached to a host file or program; it may infect other files. | Often spreads when an infected file or program is run or shared. |
| Worm | Designed to replicate and spread between systems without needing to disguise itself as a useful app. | Can spread automatically, often across networks or through vulnerabilities. |
This is a high-level distinction, not a complete taxonomy: malware can combine techniques or install other types of malware. The important practical point is that a Trojan’s disguise and the decision to install it are central to its typical route onto a device.
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What can a Trojan do after installation?
There is no single “Trojan behavior.” Depending on the malware, it may steal passwords or sign-in details, record keystrokes or browsing, download additional malware, facilitate fraud, or let an attacker control the device. Microsoft lists these as possible Trojan capabilities, not guaranteed effects of every infection. Microsoft’s explanation of Trojan behavior provides further detail.
- Credential theft: Information entered on the device, including passwords, may be captured or transmitted.
- Surveillance: Some Trojans can record keystrokes or visited sites.
- Follow-on malware: A Trojan may download or install additional malicious software.
- Fraud or unauthorized access: Some enable fraudulent activity or give an attacker control of the device.
A slow computer, unexpected pop-ups, or another isolated problem does not prove a Trojan is present. Those symptoms can have many causes, and they are not a reliable diagnosis by themselves. Use a reputable security scan and official recovery guidance rather than inferring infection from one symptom.
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How do Trojans get installed?
The common thread is deception: a malicious file or app looks like something the user wants or trusts, and the user installs or runs it. Another malware program can also deliver and install a Trojan. That means prevention is not only about spotting a suspicious filename; it is also about choosing trustworthy sources and pausing before granting an unfamiliar app access or running an unexpected download.
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- Download apps from trusted sources. Avoid installers from unfamiliar sites, unexpected messages, or links that pressure you to act quickly.
- Check what you are installing. Confirm the app’s source and purpose before running it, and be cautious if the requested access seems unrelated to what the app does.
- Keep security protection current. Microsoft recommends regularly updated antivirus protection and identifies Defender Antivirus as built into modern Windows versions. The FTC also advises using reputable security software configured to update automatically. See the Windows Security guidance and the FTC’s malware prevention and response advice.
- Keep backups you can recover. A separate backup can help you restore files if recovery requires a clean installation. A backup drive is storage, not malware detection or removal; scan files before restoring them.
How do you check for and remove a suspected Trojan?
On Windows, Microsoft identifies Defender Antivirus and Microsoft Safety Scanner as free tools for detecting and removing malware. Use the current official instructions for your Windows version and the chosen tool; no scan can be assumed to find or remove every threat. Microsoft’s Safety Scanner page explains the tool, and its Windows Security instructions cover built-in protection.
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- Use a trusted device for sensitive account changes. If you think passwords may have been exposed, do not change them on the possibly compromised computer. Use a different, trusted device to change important passwords, starting with email and financial accounts. The FTC specifically advises changing passwords from another computer when malware may be present. FTC malware guidance.
- Run an up-to-date security scan. Follow the official instructions for Defender Antivirus or Microsoft Safety Scanner, and review any detection and removal results.
- Secure affected accounts. Change reused passwords as well as passwords for accounts that may have been accessed, and use each account’s official recovery process if you cannot sign in.
- Restore files cautiously. If you use backups, scan files before restoring them so you do not reintroduce malicious files.
- Escalate if the malware remains. If security software cannot remove malicious files, consult current guidance from your device maker or IT support. Wiping the device and reinstalling its operating system can be a last-resort recovery option, but may erase programs and personal files. Back up recoverable data and understand the device-specific steps before proceeding. The government’s incident-handling guidance discusses backup and recovery precautions, including scanning files before restoring them.
What does the “62%” Trojan statistic mean?
Microsoft’s 2024 publication of Windows Malicious Software Removal Tool: Progress Made, Trends Observed reports that a backdoor Trojan was present on 62% of 5.7 million unique computers from which Microsoft’s Malicious Software Removal Tool removed malware during the historical period from January 2005 to February 2006. That figure describes an old, tool-selected group of infected Windows computers—not all computers, all users, or today’s infection rate. It should not be used as a current estimate of Trojan prevalence. Microsoft’s report provides the historical context.
No current, geographically representative Trojan infection prevalence figure is established by the sources cited here. The historical statistic cannot fill that gap.
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