Microsoft's OneCare Live OneCare is actually available for purchase. I used it for a while. It's lighter on the processor that Norton Systemworks. It doesn't do as much, but it still provides Anti-Spam, Antivirus, and does "System Tune-Ups", which remove cache files, temp files, defrag the hard drive, etc.
note:
Vista version is
here. TrendMicro's Housecall Housecall is the online scanner I use the most at client sites. It's pretty lightweight, scans well, and doesn't screw up your current antivirus. I've scanned a ton of machines and servers with this program, and it's a good compliment to whatever security solution you have. Personally, I scan my PCs once a week with this, just in case my other scanner missed something.
KasperSky's Online Scanner
I've only used
KasperSky like five times, but it did really, really well. I use it when there are really tough worms or spyware on the machine, usually directly after TrendMicro's scanner. KasperSky has been in antivirus for a while, but at the corporate level, not the end-user. Great product.
Symantec Security Check
A great tool for removal of both Viruses and Spyware.
Symantec has been tops for years, and this product/site is no exception. This link takes you to a button that will open up a new window and scan from there. Highly recommended.
Panda Software ActiveScan
The actual
Panda software is super-lightweight on your system. It won't really bulk up and take a ton of resources at all. It doesn't have a ton of features, but sometimes you just need to scan. Their online scanner does a pretty good job as well.
There are some others out there, but with the above links you can get rid of pretty much anything.