Virus scanners operate by searching your harddisk(s) for files which might be infected by a virus. This method is never 100% secure, since only known viruses (or known virus patterns) are detected. If an infection has been detected in a certain file, the virus scanner might be able to clean the file, but very often this is not the case. In that case, you can try another scanner to clean the file, you can try to replace the file or - as a last resort - delete it.

However, if many important operating system files are affected, it might be the best solution to reformat the harddisk and install the operating system from scratch.

There are even viruses can not be cleaned by mere hard-disk reformatting. E.g. so-called boot-viruses infect the boot-sector of disks, and will very often not be removed by simply reformatting the harddisk. In this case you will need to repair the boot-sector, a good utility for this can be found in Norton Utilities (not free).

Freeware lite version of the commercial Antidote program. It utilizes the same virus database as the commercial version.