While Spybot and Adaware are still fairly useful, it's possible you have stuff they're not picking up. Try downloading AVG antispyware, 30 day trial then resident shield stops. At that point you should reconfigure it to be a demand scanner. One of the beauties of this application is that when doing a memory scan only, which doesn't take long, it releases or frees up used memory that is no longer required. Don't ask me for a more technical explanation, I've read reports that it can help in this sort of situation. And if parasites are the problem, it's likely to find them. (Other good antispyware applications include Asquared, Superantispyware, and SpywareTerminator.)
When you say virtually nothing in start up menu, do you mean the start menu available when clicking start>programs, or the one in msconfig?
Have you defragged?
What percentage of the disk is used?