AVG comes to mind, it's a freebie. You can download AVG free from here:
http://free.avg.com/ww-en/download-file-ins-afeAfter installing it on the standalone computer, you can download the manual update file here:
http://download.avgfree.com/softw/80free/update/u7avi1721zp.binThen once you've installed it, copy the update file to the infected computer, open AVG, and go to Tools > Update from directory. You can then point it to the .bin update file you downloaded.
Also, you could try just booting to an offline CD or USB media, and use a standalone scanner that way.. There are a lot of options, but you could try one of the following free utilities.
Ultimate Boot CD -
http://www.ultimatebootcd.comDr Web Standalone Scanner -
ftp://ftp.drweb.com/pub/drweb/livecd/20091014042001/minDrWebLiveCD-5.0.0.isoHope this helps!