Az gave you the three best choices. They are all free and ad aware is one of the best. One thing you might consider though is once done that should also delete you temp files.

To do so with internet explorer open.
-Click tools/ at top of screen
-then Internet options.
-Press button deleat cookies,
-after that done press button deleat temp files. At this time will get a pop up and put check mark in to deleate all off line content. Depending how long it has been can take a whle but you will know working if you see the hard drive light flashing on case.
-Then press ok.
-After that then press settings and were it says amount of disk space to use ( lower to 10MB) then press ok.

If you are still having slow computer then should do a msconfig. If need info add to this topic and will give the info.