My first question would be "Why do you think you would need to write a scanner?" There are already a number available, and I don't think the world really needs another one all that desperately. There are hundreds of viruses, worms and other nasties out there, and I don't think that you'd easily find anyone willing to give you the data you ask for.
I'd like to suggest that you explore other approaches to defending against malware than scanners -- that is a fairly mature field already.
For more information on this topic, visit these other SearchSecurity.com resources:
Virus Prevention Tip: Exposing the myth of antivirus
Scheier's Security Product Roundup: Antivirus tools unify and conquer
Virus Prevention Tip: Antivirus hardware?
This was first published in October 2002