Many people say you *need* a real-time anti-virus program on your computer that constantly checks for virus infections every time you launch a program or load a document. I say: real time virus scan software can cause almost as much slowdown and crashes as the virus/malware you are trying to protect against. Better to run the scanner just once (on demand) - when you first download a new program. You save on computer resources, and avoid compatibility issues.
Bit Defender offers a free version of their anti-virus/malware scanner that only runs on demand - the big 'feature' they are offering in the non-free version is real-time protection. The download link is here. Note that they give you the hard sell - you have to repeatedly confirm that you really want the free version. A bad sign.
The installation is pretty invasive. Several programs are loaded into memory even when you aren't running a scan - the whole point of on-demand scanning is to avoid this kind of bloat! Also, an item is added to your context menu in explorer, with no clear way to turn it off.
When you start the program it also takes the opportunity to remind you that it's only the free version, and that you should upgrade. Well, that's fair game for any shareware app, but it is annoying, given that other free scanners don't do that. To top it off - the free program expires (in about 100 days, in my case), after which you must reinstall again. I don't think hassling users is a good way to get them to upgrade to the full version.
All in all, it seems that Bit Defender Free Edition is a reasonable choice for cleaning out a known infection, but is too annoying to regularly use as a on-demand scanner. The context menu does make it very easy to scan a file on demand, but at the cost of making your menu all that much more bloated. Give me a "send to" option, or let me drag and drop the file I want to scan. Within the main program have an option where you can select a particular folder for scanning (say, your download folder), and save that as a pre-defined 'task' you can activate with one click next time you start the program. That would be a fine option if didn't insist on nagging you about registering and loading needless programs into memory even when not scanning.
On the whole, I'm unimpressed. HouseCall seems to be a better option; it has fewer options, but works more or less how you want right out of the box, and isn't so annoying.