It might be time to do a repair installation of Windows to rescue stuff.
how to do a repair installation of Windows and you want Method #2 there (it's about 3/4 of the way down the page).
This should get you operational enough to rescue stuff and save it onto an external drive or DVDs. Once you've got your stuff backed up, then you can nuke your existing Windows installation completely and start from scratch.
To completely remove the existing Windows installation, one method would be to run Windows Setup by booting from the Windows CD, use Windows Setup to
delete all existing partitions on the hard drive, then press the
F3 key twice to
exit from Windows Setup. After that, start Windows Setup a second time, create a partition, quick-format it with NTFS, and keep going.
When reinstalling Windows, you want firewall protection. If you scroll halfway down
this page, there's instructions on how to keep network worms from infecting the system when it's freshly installed and vulnerable to attack.
Good luck, and if you can burn the F-Secure Rescue CD and run it before doing your backup, it's less likely that your backup will contain malware.