Assuming your PC is malware free, and performance is not being affected by insufficient RAM (and therefore excessive paging) then applications loading at startup is where you need to look.
Startup Inspector is pretty good and makes recommendations on what can/should/shouldn't be disabled:
http://www.windowsstartup.com/
The most comprehensive I have found is Autoruns from Sysinternals, however this requires more expertise than Startup Inspector:
http://technet.microsoft.com/en-us/s.../bb963902.aspx
Ed.