In XP you can use the system Administrator account in Safe Mode by tapping Function 8 as you power up the machine. Go for the Safe Mode with networking option so you can download malware trackers.
If you then go from Start to Run you can type msconfig into the Open box and click on the StartUp tab then untick anything you don't want to run at StartUp. Unfortunately, that doesn't stop everything. Which threat do you think you had?