A virus scanner is no longer an optional utility to have running on your computer. Its absolutely essential considering the vast number of viruses in circulation that threaten us daily.
The problem is that once a virus scanner is installed, there is no way of knowing if the software actually works. You can test it by finding a virus and copying it to your hard disk, but most people would be somewhat hesitant with installing software that could potentially destroy their computer.
An alternate and safe method of testing the scanner is with the EICAR test file. It was developed by the European Institute for Computer Antivirus Research for testing virus scanners. This program isn’t a real virus, but every virus scanner (at least the major ones) have it in their virus definition file and it will trip the scanner. Once detected, the virus scanner will either delete or quarantine it depending on its settings.
You can even run the program from the command prompt which will display a string describing the program.
I’ve made the EICAR test file available here. The zip file has a password (eicar) to prevent a virus scanner from reading the archive and then deleting the file inside.
A tutorial on how it works (which includes an introduction to x86 assembly language) can be found at An Examination of the EICAR’s Standard A-V Test Program page.