It could be that you have too many programs starting up and running in the back ground all the time.
You don't say which system you are running (XP? Win 7?) but try this:-
Click START, RUN, type MSCONFIG in the panel and then click OK.
Click the START tab (top right) and see what programs you have running at Start-up.
If you know what any program is and you don't need it running all the time then remove the tick from it's box.
DON'T remove any ticks for programs if you don't know what they are or what they do as you could make things a whole lot worse.
Finally, click APPLY and then OK.
If you have changed anything at all then the next time you use the computer a panel will tell you that things have been changed, tick the box to tell it not to advise you any more.
Hope this helps.