I've had good results with the ESET online scanner (the NOD32 folks). It can be found here:

www.eset.com/us/online-scanner

Yes, it will make a half-assed attempt to sell you the retail package but it wasn't not a major nag about it last time I used it.

Normally, that's what I would do, but in this case it's a VPN IP that's flagged as having had something weeks earlier before I started using that exit point, and manually looking for the connection found nothing. On top of which, I run noscript, flashblock and a few other security extensions which really reduce the likelihood of a drive by.

In other words, I'm pretty damn certain that I don't have anything on my disk, but it never hurts to do an extra scan.