It’s probably been blogged a lot by now but I just wanted to mention VirusTotal and jotti.org. They both offer online virus scanning of single files (or archives) of up to 10 megs. The difference between them and other online scans is that they run just about every virus scanner and malware scanner going on the files, and report back results from all of them.
Sometimes you get a few false positives, but they also pick up a lot of stuff that whatever AV software you run may miss. Unfortunately no one AV program detects all viruses, and this is a free way of running just about every tool going on a suspicious file. There is even a handy program you can download from the VirusTotal web site which adds Send To->VirusTotal to the right-click menu in explorer. You can submit files by email as well.