Weird virus. Scans can't detect it.

Answer 1

TrendMicro HouseCall is very good, but if nothing is found try downloading SuperAntiSpyware and MalwareBytes Anti Malware.  Update them both then use SuperAntiSpyware's BootSafe with networking and have both programs scan your external HDD.  One will ask you to reboot after the scan, don't but run the other instead.  This may take a couple of passes but both programs find a lot and I've yet to have a problem after they've finished and I've used them both to clean many computers.