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Remove viruses from an infected PC, and keep them from coming back

Clearing malware out of a system is only half the battle; you also have to prevent reinfection.

Dennis O'Reilly Former CNET contributor
Dennis O'Reilly began writing about workplace technology as an editor for Ziff-Davis' Computer Select, back when CDs were new-fangled, and IBM's PC XT was wowing the crowds at Comdex. He spent more than seven years running PC World's award-winning Here's How section, beginning in 2000. O'Reilly has written about everything from web search to PC security to Microsoft Excel customizations. Along with designing, building, and managing several different web sites, Dennis created the Travel Reference Library, a database of travel guidebook reviews that was converted to the web in 1996 and operated through 2000.
Dennis O'Reilly
3 min read

Our family PC gets quite a workout. It's a five-year-old machine that runs Windows XP and is used primarily by my daughter and teenage grandson for instant messaging, e-mail, social networking, and downloading audio and video files. Since I rarely use the system, I didn't notice that its antivirus subscription had expired.

Which explains why I was a bit surprised when my grandson called when I was out of town to tell me that the PC was acting strangely. Ads appeared on the desktop as soon as Windows started and Firefox and other programs would occasionally close without warning or fail to open at all.

I immediately suspected a virus and instructed my grandson to perform a virus scan. Unfortunately, the machine's antivirus app had gone AWOL. I talked him through the process of using System Restore to revert the PC to an earlier time. This improved matters somewhat, but the system continued to act flaky.

When I returned from the trip, I started the troublesome machine and attempted to open the Microsoft Update site to make sure its copy of XP was up-to-date. But the malware had managed to disable several Windows services intermittently, including Services.msc, so Internet Explorer would shut down repeatedly.

At this point, I was seriously considering a hard-disk reformat and XP reinstall. I even had the XP installation CD in the drive and was ready to begin the process. But even though my daughter and grandson assured me that they had backup copies of all their personal files, I decided to try one more time to salvage the existing setup.

I'm very glad I did, because it turns out there were lots of vacation and holiday images and videos on the machine that hadn't been backed up. First, I installed a free copy of Malwarebytes' Anti-Malware antivirus program on the infected PC, updated the app's virus definitions, and ran a complete scan.

Malwarebytes Anti-Malware scan report
The initial Malwarebytes Anti-Malware scan detected 104 separate infected files and folders. Malwarebytes

That first scan turned up a mere 104 infected files and folders. Here's a list of the nasties the machine had picked up:

• Trojan.Vundo
• Troja.Vundo.H
• Trojan.FakeAlert
• Rogue.Installer
• Trojan.Downloader
• Trojan. Dropper
• Trojan.Agent
• Worm.KoobFace
• Rogue.AdvancedVirusRemover
• Rogue.SystemSecurity
• Adware.BHO
• Rootkit.Agent
• Spyware.Agent
• Trojan.BHO
• Hijack.LSP
• Rogue.Multiple
• Disabled.Security

After viewing the report, I rebooted the PC and ran another malware scan. This time, Malwarebytes' app found only nine infected files.

The second Malwarebytes Anti-Malware scan report
The second Malwarebytes Anti-Malware scan detected only nine infected items. Malwarebytes

I rebooted once more and ran yet another scan, which indicated that the PC came up clean.

The third Malwarebytes Anti-Malware scan report
The third Malwarebytes Anti-Malware scan indicated that all viruses and other malware had been removed from the infected PC. Malwarebytes

Once I was assured that the PC was malware-free, I revisited the Microsoft Update site to download and install all the XP security patches the machine required. Then I sprang for the $25 version of Anti-Malware to get the program's real-time virus scanning and automatic updates.

I knew all attempts to alter the user behavior that led to the infections would be futile, so instead, I instructed my daughter and grandson to run Malwarebyte's scanner each time they start the system and just before each shutdown. That was a little over two weeks ago, and so far, the PC remains free of infection. Still, you can bet I'll be paying much closer attention to that machine from now on.