Often, even if we do catch a virus, it's not so difficult to eradicate it using installed anti-virus—but if your system has been crippled, try using Linux to scan the drive for viruses instead.
As any Linux veteran knows, one of Linux's greatest uses is fixing unbootable drives—recovering files, deleting files, and even killing viruses. For those of you that aren't quite as well-versed in Linux, technology blog gHacks has a tutorial for doing just that, though we recommend a few tweaks to their process.
If you have another machine already running Linux, as gHacks suggests, then you'll have a pretty easy time with this—all you need to do is install some antivirus, hook the infected drive into your Linux machine, and go to town. However, we know that not everyone has an extra Linux machine just floating around—so we recommend using a Live CD with antivirus pre-installed, or, even better, a live USB stick on which you can install antivirus yourself (the USB solution is likely easier in the long run, since I have yet to find a Linux Live CD with a GUI-based antivirus program pre-installed). You'll have to make the live USB yourself, but this is a pretty easy process using previously mentioned Usbuntu Live Creator or UNetbootin. However, these require a working Windows computer, and if your only PC is the infected one, you'll have to download, burn, and boot from the Ubuntu Live CD (available here), and under System > Administration, use their easy-to-use Live USB creator.
After making the Live USB stick, boot into it (you may have to set your boot priority in your BIOS, directions for which can be found in step 2 of this post), and install antivirus on it—gHacks recommends F-Prot, but if you don't want to buy a copy of F-Prot just for this, all of our Hive Five antivirus favorites have Linux versions, and open-source favourite ClamAV (pictured above) is available from Synaptic Package Manager (along with the Clamtk GUI for it—just search for clamav and clamtk in Synaptic [available under System > Administration]and install both packages). Start it up from Applications > System Tools, set it to scan your Windows drive and you're good to go.
Note that if your BIOS does not support booting from USB, you'll need to make a boot disk that allows it to—which, unfortunately, requires a bit of command-line-fu, and then you're right back where you started with the command-line-requiring Live CDs. If you know of any Linux Live CDs that contain an Antivirus program with a GUI, let us know in the comments!