When programs vanish from your PC or laptop, there's every chance you've been hit by malware. When a system scan found and removed a virus from one of our readers' PCs, it was only the start of his problems. Our Helproom Expert showed him how to use the System File Checker to get things back to normal.

QUESTION Several programs vanished from my PC, including AVG FREE, Windows Firewall, ZoneAlarm Firewall and Microsoft Security Essentials. I've managed to reinstall only the latter two services. A full system scan detected and removed a virus, and ZoneAlarm now reports that everything is working as it should. However, System Restore refuses to restore to an earlier point or create any new ones unless I restart the PC, yet doing so returns the same error message. A Hall

HELPROOM ANSWER It sounds as though you're experiencing the aftermath of a malware infection. Although your antivirus software has probably removed the offending program, some of its damaging effects seem to still be in place.

It's not a good idea to roll back your machine to a previous point following a malware infection, since System Restore's archive may include infected files. You can delete these files, along with all previous restore points, by turning off System Restore and turning it back on again.

Use the System File Checker to restore any system files that may have been deleted or corrupted by the malware. Click Start and type cmd in the Search box. Right-click the ‘cmd' or ‘cmd.exe' entry that's returned and choose ‘Run as Administrator'. This will open a new command prompt with the word ‘Administrator' displayed at the top left.

Type SFC /SCANNOW and hit Enter. This process may take several minutes depending on the speed of your computer. If there are any files that need to be reinstalled, the utility may ask you to insert your Windows disc – ensure you have it to hand.

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