Avast! Virus Cleaner is a tiny utility to help combat viruses and worms. Please keep in mind that it is not meant to be used as a computer's sole defense against viruses, it is merely a supplement, mainly for if your computer already has a worm.
It's hard to say anything about this, because it's very straightforward. The program is standalone, 400kb in size, 2.5mb RAM on both idle and scanning, and completely portable. Press the "Start Scanning" button and it will start scanning your computer for some 20 different worm types (please check the Avast! website at the bottom of the post for more info), and a box shows the normal virus scan stuff: current operation, current file, number of infections, etc.
To be honest, I really can't say much about it, just because I've never had to use it. My computers in the past have either (a) been protected enough from viruses that it never came to having to use this program, or (b) got so toasted by virus(es) that I had to start with a fresh installation. So I cannot give my testimony about Avast! Virus Cleaner, but I'll do my best to review it, with my limited virus-related knowledge.
First off, I do have to say that A!VC will immediately scan your memory on the start of the scan. That means that if a virus is running, it will either kill it, or at least disable it, so it will stop from spreading. Then it scans your disks, then your startup, then the infected files are dealt with, then your system is clean and happy (or at least it should be).
The last thing I am noticing on the page is that you can "run Avast! Virus Cleaner 'somehow'", meaning that it doesn't have to be run as an EXE. Other filetypes can be run, such as BAT, PIF, COM, and SCR, or even CMD. If a virus disables your ability to run EXEs, you can just rename A!VCT to one of the named filetypes above and try that. It's like outsmarting a computer virus! The only reason I mention this is because I have indeed witnessed a virus taking out all the EXEs on my computer; every time I clicked the start button, more icons would go missing. If I had known about A!VC back then, it probably would have saved me from doing a System Restore.
The thing about A!VC is that it's really something that you need to have on hand. Odds are you might not be thinking about what to do if you get a virus until you actually have one, but A!VC is a handy, small tool to keep as an ace in the hole, so you aren't freaking out about what you should download before your machine is toast.
Remember, I've never used A!VC (well, I've scanned with it, but it's never found anything), so I cannot say for certain if it works, but it is from Alwil Software, so it is at least safe, and for how small it is, it is a very handy tool to have on hand.
*If someone ever wants to run a controlled experiment and download a virus and use A!VC and tell me if it works, be my guest. Please note "controlled"; I'm not telling you to go downloading viruses willy-nilly. I take no responsibility for such foolishness you might commit.