You can take everything off the startup list and the machine will still work - I have just 3 entries on my list...
Spybot's TeaTimer
Apoint - Alp's pointing device driver
Microsoft's Cftmon - which I could easily turn off.
Any security S/W should ideally be left on the list but it's OK to take it off for test purposes
When my system began to behave like yours recently I used task Manager to see what was eating the Machine Resources - You might also consider using a free utility that paints prettier pictures - System Internals' Process Explorer.... (free)...
The culprit was svchost.exe consuming close to 100% of CPU time and dropping sometimes to 93%.
I cured the problem by returning to a restore point - it has been OK for the past 3 or 4 months now.
You really should look to see where all the machine cycles are going - something is very probably gobbling them up. Once you know what it is then you can take the appropriate action - that or just return to a restore point whithout bothering to look.
Good luck...
Peccavi
May 2010