Well this isn't exactly free...
For those Dr. Web users out there... you might know that Dr. Web uses the AVP engine (which isn't exactly the fastest engine out there)... But I stumbled upon a program I knew I should not have missed...
Its called adinf32. I cannot believe I missed this one... but adinf can plugin to Dr. Web. Adinf is an integrity checker that checks if a file has been scanned already by Dr. Web. If it is, then it goes on to the other file. If the file has been modified or is new.. then Dr. Web scans it with it. (This is similiar to Sophos's Intercheck technology) The result? A much much faster system. You can also recover from a virus easily with Adinf.
http://www.adinf.com/ http://www.sald.com/