No antivirus program can catch every single virus out there. And if the virus is running while you are trying to install an antivirus program to remove it. Well, YMMV.
The easiest way is to remove the hard disk, attach it to another computer, and scan it from there. Taking care not to infect the second PC as well.

But many people do not do this.
The surest way to get rid of the virus is to reformat the hard disk and reinstall everything. This is what will happen if you bring your computer to a shop or IT dept for repair.
Once you get rid of the virus, you should format your hard disk into at least 2 partitions. One for Windows and programs. Another for your data. This way, it's easier to backup your data, and when you have to reformat c: again in the future, you don't loose all your data. Copying to d: the few files you have in "My Documents", IE favourites, etc, should be easy.
Once your computer is cleaned, make a complete backup of c:. I regularly make a full backup of c: every few months. This way, if I do not need to reinstall everything (which is troublesome as you need to look for drivers, programs, configure lots of things, etc). Just restore your backup. I use a free program call DriveImageXML.
But recently, I've switched to using PartImage in Linux. I have a relatively small 100MB partition with Puppy Linux, and a script to run PartImage to backup or restore the windows c: drive. I only need to reboot the computer, press 2 keys, and 3 minutes later, the restore is complete. Because of this, I don't run antivirus software. I like my computer to be as fast as possible.
This post has been edited by ChayimT: Oct 6 2008, 03:16 AM