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.
MalwareBytes
This is one of the top Malware Scanners anywhere. This is a good first step in scanning for malware and baddies that show up and gank your system. They're on my short list of items to always have on you.
(reminded to add this by Hunty)
Some more Malware Removal Tools
CCleaner
CCleaner (formerly Crap Cleaner) is a great utility that erases your temp files, cookies, cache, and all unnecessary junk from your machine. Run it once on your PC with default settings, then log on as every user that has logged onto that machine and run it again for maximum cleaning.
SuperAntispyware
Great tool, that again, is easy to install and use the default settings. The free version won't let you run updates or scans automatically, but is good for the occasional scan. The one setting you need to change is turning off the "scan only known filetypes" in the settings tab so it scans absolutely everything. This is probably my second most-used tool in my box.
Crazy Closet
Parachute Panic
Snake
Chopper Challenge
Curveball
Mouse Race
Linear Mode
