I need to develop antivirus software for Windows. Where can I download viruses or virus-like programs to check it with?
The EICAR test file is a good start, as it is an example of a self-updating program - something that malware usually does.
Try using one of many fake malware protection programs. Some SO-scrapers are "mapped" recently, links are in the meta-stack overflow.
Perhaps make sure you are trying to use them in a virtual machine disconnected from the Internet .;)