I guess, you should come up with all details like OS, Configuration and all. Few things I can suggest:-
1. Run default Cleaner/Defragmenter, Boot time scan and normal scan using latest and updated a.virus program.
2. Go "cmd" type "msconfig" and select "start up" tab, Un-mark all the thread other then Antivirus. Restart!
3. You can clean and clear cache/temp and cookies.
4. Go "cmd" type one by one "prefetch/temp/recent" new windows will appear every time. Select all and delete them all.
5. Check out "Task Manager" Process. Make sure there is few process running and you know them well. If any thing new or strange just End them, It may be virus/Trojan/worm/stupid thread to eat up your memory.
6. you may use "Ccleaner"
7. Be sure you have enough hard disk space and RAM support.
8. Get rid of Malware/Spyware if there is any.
9. Check out Power Management in Control Panel.
10. Be sure about video/graphics cards and Display options.
"People says 143. I say 444 i.e. Love Your Life"