Make a new user account and run rkill, then disable the proxy in the windows control panel and change your DNS settings to auomatic. Install an antispyware program fresh and scan with it, then run hijackthis and let hijackthis.de analyze your log, pick out whatever in there it marks as bad and anything else you find suspicious (like shit with a random name running from application data or a temp directory, keep a specific eye out for those) and fix it, then do a full antivirus scan.
If critical windows files have become infected you're kinda' fucked (and when this is fixed you need to turn UAC on and leave it on), you'll need to borrow somebody else's disc (with vista the version doesn't matter as long as it's the right bit-depth) and do an in-place "upgrade" (which is like XP's repair install), which will reinstall windows overtop of itself, leaving all your personal files, but some/all programs need to be reinstalled and windows update needs to be run again.