When you internet explorer is open - click:
Tools -> Internet Options
About halfway down the "General tab" - there should be a "Temporary internet files" box - click on "Delete Files" and when the next box comes uo - click on "Delete offline" files.
If you haven't done it before, it may take a bit to clean up the junk so don't be worried.
Only clean the "Cookie" one if you are sure you have the passwords and accounts to everything that you have "autofilled" when you go to pages.
Also, give this program a try if you haven't cleaned your pc up before:http://www.safer-networking.org/en/download/
It won't get rid of the temporary internet files (like I described above) but it will clean any spyware that you have collected. PS. You don't have to donate any $ to spybot if you don't want to - it is a freeware.
Both steps will potentially speed your pc up a bit.