If you just want a "second opinion" demand scanner, have a look at
http://download.drweb.com/drweb+cureit/ It's a 5~Mb download, standalone application. You run it from the download location; there is no installation, nor updating. To update, a new 5Mb application must be downloaded.
I've tried this, and Clam, and online scans while using Avast. None ever find a thing.
Be careful about trying to have too many security applications running. (Resident/realtime).
General rule is 1 AV. 1 Firewall (you can't have more than one.) and (optional but recommended these days) 1 Antispy/antitrojan.
You can have as many demand scanners as you want, two or three is probably more than enough, though.