I believe the easiest is to check to see if the registry entries for a
particular antivirus package exist...
Since the is information avaliable from the security control center of WinXP
SP2 or so, there is possibly other more official way to query it, but from
the perspective of not letting virus writer to have way to identify
antivirus programs(even non-famous ones), they may not publish it. I have no
idea...