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. A number of virus scanners listed on this page is able to repair a boot-sector.

Free Online Virus & Trojan Scanners
Freeware online Trojan scanner. This program runs in your browser (Internet Explorer only, Windows), no explicit installation is required. Freeware online virus scanner. You can upload a single file which will be scanned by around 20 virus scanners.

Non-free Virus Scanners
The programs below you can try out for free for a certain time. During this trial period they can be very helpful. You need to spend a little bit of money should you decide to use them beyond that period.

Free Virus Prevention
AnalogX Script Defender protects against script viruses (those viruses using e.g. Visual Basic Script, Java Script). Gives a warning when scripts are executed. If you still use floppies, then this handy freeware utility might prove invaluable. Bootminder prevents your computer from being infected by boot-viruses. Bootminder warns you when your A: drive contains a floppy just before Windows shutdown. This way, you won't start up your computer the next time with a potentially infected floppy still in it. Result: no boot-virus infection. Freeware for Windows