What system are you using, XP?
Things to do in order and assuming you are covered with up to date AV and Firewall.
Download and run CCleaner.
If you recognise something is installed which you don't use, go control panel > add/remove programs, and use that facility.
Have a look in Start > Run >msconfig and choose the right hand tab.
This will show all the programs which load at startup. Untick the box beside any you know you don't need for this session. (they can always be opened normally if and when you need them).
Run CClean again and see if that improves things.