Without a better description of your computer it's difficult to tell.
Common causes are :-
1) Hard drive full - delete/ defragement
2) Too many running programs for your current memory
3) Viruses - Please tell me you have a virus scanner.
4) too many running items.
Every thing you have running simultaneously uses memory and CPU time. Check your CPU and memory commitments by pressing ctrl/alt together and then DELETE (not backspace) to bring up task manager. Click on Performance tab to see at the top your CPU usage and at the bottom your memory usage. IF CPU and Memory high - you've got something that's eating your processor up.
pcdocmail@talktalk.net, October 2006