Steps I'd recommend...
1) Buy a USB thumbdrive from your local computer shop or Walmart.
2) Plug it into your machine and copy your personal files over (noting that these may be contaminated by your lingering viruses)
3) Remove the USB tumbdrive and turn the PC off
4) Get a copy of the latest Ubuntu LiveCD.
5) Boot the PC off the LiveCD, and when you are given the option to do so...
6) Delete your partitions and reformat the drive as EXT4 (i.e. let Ubuntu take full control of the C drive and incidentally wipe out any viruses and damage on that drive)
7) Plug your USB and copy the files over to a directory in your new home folder
Erase the USB stick so no one can catch a virus off of it.
9) Wait a few weeks and grab some antivirus software from the Ubuntu repositories.
10) Scan your new PC...the viruses it detects are 99.9999% guaranteed to not be able to harm your PC while it's running Ubuntu, but you want to make your computer safe for the other machines on the network....no use infecting a friend, right?
11) Keep Ubuntu on your PC, it will make it harder for this situation to recur in the future.
I know, the above sounds like I'm nothing more than an Ubuntu fanboy, but after making the switch myself (needed a new OS and wasn't willing to burn $300 USD for it) I am quite happy, overall. It takes a little getting used to, but it's far more stable, uses less resources, and is far more resistant to PC viruses. Virus writers target the Windows platform more often than not, and Ubuntu is setup to make it difficult to sneak anything into your system that you are unaware of in the first place.
Good luck, friend!
