Have you tried the usual things that may (or may not!) help but certainly would be good things to do in any case:
Clear cache, cookies and temporary files.
Defrag all hard drives and partitions.
Run spyware and trojan hunters.
I suggest you use Ewido, A squared, X-cleaner, spybot and ad-aware. They are all free.
Which firewall do you use? Norton could be responsible for your problems.
When your computer has finally booted and before you open any programmes, have a look at your task manager. How many processes are running?