This is a very simple guide on how to fix your Windows partition if you can't access it due to virii (or because a virus is blocking you from browsing antivirus websites or updating your definitions).
It might also happen that the virus (for example TDSS) installs itself as a rootkit in the hard drive's boot sector. I've seen this in a few friends' PCs. If this is the case, it might be VERY difficult to get rid of the virus while running windows from that particular computer.
The solution? Running a Live CD of another OS to wipe your hard drive clean of virii.
My first suggestion would be going full open-source and fully functional OS. So, Ubuntu (probably THE most hardware-friendly OS out there) and ClamAV with the latest definitions (that could be updated while running the live OS). You can download the latest Ubuntu CD and then boot it, download ClamAV and then run it, but I'll give you simpler solutions.
Step 1) Download the ClamAV live CD This is a very useful set of tools that include a Windows password removal tool and testdisk.
Step 2) If you're planning on booting from a usb-stick (pen drive) and not from a CD, get UNetbootin too.
Step 3) Put in the Live CD or pen drive and boot it! ClamAV is a very good antivirus at removing infections without deleting files.
If that din't solve it, you can try running the Kaspersky Rescue CD (also works with UNetbootin) which is old, but very good nonetheless.
An honorable mention goes to "Trinity Rescue Kit" that also has lots of tools and antivirus applications to fix your virus-ridden PC.