How can I test the antivirus program I am developing?

Question

I need to develop antivirus software for Windows. Where can I download viruses or virus-like programs to test it with?

Answer 1

The EICAR test file is a good start, as it is an example of a self modifying program -- something that malware usually does.

Answer 2

Try one of the many fake antimalware programs out there. Some SO scrapers "featured" one recently, links are in meta stack overflow.

Possibly, make sure you try them in a virtual machine disconnected from the internet. ;)