You may have often read on forums ... NEVER install multiple AV programs. While it's true that you never want to have multiple AV programs **active**, here is certainly nothing wrong with having a second program around in passive mode. In other words, when in passive mode, an AV program doesn't do anything except what you manually initiate. It doesn't check each files you open, it doesn't check downloads automatically ... what it does do is what you tell it to do. So if you tell it to "Scan drive D:\, it will scan drive D:\. If you tell it to scan a particular file or folder, it will do that. If you tell it to scan every night while you are sleeping, it does that.
Other options you may undertake are:
a) If you have multiple PCs, you can put 1 program on PC # 1 and another on PC #2 ... at night, have them scan each other's HDs.
b) Ina situation like yours, and this is easier if you have slide out drive trays, pop put the HD and stick it in a HD dock like the BlacX
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Order=BESTMATCH&Description=blacx&N=-1&isNodeId=1
This is how I generally clean other's infected HDs. Drop that suspect HD into the dock, and have any computer run a scan on it.
Zone Alarm, Kapersky, BitDefender all have fully functional 30 day free trials which you can d/l for this purpose. But again, if installing as an active scanner, make sure to disable your current AV when running. Also when you download the trials, you should d/l the vendor's "uninstall Tool" and run as a check after uninstalling, to make sure everything got removed.