For the first time in Vista, AVG found a virus this week. Or... it was a trojan in the DLL and EXE for a popular install program, so I had several copies scattered in Temp folders going back six months to the modem driver install. That seemed unlikely.
There are several online virus checkers for suspicious files. I tried a couple and the files came up clean, so this was an AVG only alert.
I typed the name AVG came up with into Google, and several forums were discussing Generic6.UMS already. It looked to be a 'false positive', an error which we'd all downloaded with that day's AVG auto update, and sure enough the next day's update removed it.
So AVG working correctly then, and this is the first time I've had it do this. I expect there are lots of reformatted hard drives and new installs out there now!