One of the problems that many computer users experience after a computer virus attack are changes to the system cause by the malicious software. This can for instance mean that system tools like the control panel or task manager do not open, that security permissions have changed, services or startup items have been added or modified or that processes run in the background that have been loaded by the virus. It is sometimes impossible to fix the damage done to the system which means that in those cases only backups or new installations are an option

Excerpt from:
Recover Operating System After Virus Attack