Today I consider a free On Demand anti-virus program: Trend Micro HouseCall (7.1 beta). This program will scan for spyware and viruses on the folder or disk drive of your choice, and each time you run it, it automatically checks for new updates. Since it's from a major Anti-Virus vendor, it's likely to be updated very frequently. And the interface, as you can see, is very simple. To install, you download HousecallLauncher.exe, which is deceptively small, and actually creates an installation in your Windows Temp dir, and downloads the latest anti-virus signatures. This installation seems to persist after you close the program (nothing stays resident in RAM, however).
Scanning is very fast if you select the default option, but this doesn't check all of your installed programs, so it's a bit misleading. A full scan checks every single file on your computer (independent of file time), and takes a really long time. For on-demand use, select the custom option, and then the directory where the newly downloaded file lives. The program can search inside ZIP files; I don't know about other archive types.
As an on-demand scanner, it's very reasonable, but not 100% optimal, for two reasons. First, it takes a while to load, and second, you have to navigate through the gui to select the target folder to scan (no drag-and-drop or command line options).