Hi Round1...
What is the spec of the machine? Hard Disk Size, CPU speed, How many drives etc.. all these are a factor, on top of the amount of s/w you have installed / de-installed (some de-installs leave trace in the registry).
I take it that the machine is running Microsoft windows of some release for the OS. Copy any stuff that you no longer use / need to external backup (external hard drive, DVD, CD )... Then have a look at your c:\ - if you have a lot of stuff in "My Documents", "My Pictures", etc etc.. move them out of there... ideally the c:\ should be small (if you have more than 1 disk - either partitioned or physical 2nd disk).
Un-install any software that you no longer need or use (just make sure that you have the install CD if there is a chance that you will use the s/w again in the future). Clean out your browser history... try under temp / tmp folders... all that can be removed...
Once you have "cleaned up"... then run the machines de-fragement software... depending on the size of the disks that may take a few hours (especially if it hasn't been done before).
Also check the software that "starts up" automatically on boot up - should be able to change that from the s/w options itself or run "msconfig" from start -> run -> msconfig - just be careful with that.. if you haven't used it before, read up on it... stay away from System.ini, win.ini,boot.ini tabs... just take a look at the startup tab. Some of this software loads up on boot up & then checks for updates of itself automatically if connected to the net... takes CPU time to do this thus slows your machine.
Check your memory - most machines should be able to take at least 2Gigs of RAM - if you can get more in there, then buy more (klompett.ie are reasonable for this).
lastly, make sure you haven't got any viruses on the machine... should run a deep scan at least once a week & standard scan during the week - all depending on how much you use the web.