Have you ever emptied your "Temporary Internet Files" folder. If not, this is where you will find your biggest gain.
Open Internet Explorer 7. If you have the menu's showing, click on TOOLS, then on the pull down menu click INTERNET OPTIONS (new window opens)and then on DELETE in the BROWSING HISTORY section of this new window. Another window opens, then work your way down the various DELETE buttons, or use the DELETE ALL button at the bottom of this window if you don't have any saved passwords.
If the menu's don't show in IE7, Right click to the right of the TABs in a clear area of the TAB BAR and click on MENU BAR (puts a tick against it) in the selection box that appears
This clearing out proces might take several minutes so don't panic.
If you are still on IE6 the process is similar enough to not get lost and will still take a fair old time. (Silver Surfer's memory stops me remembering the detail for IE6)
Hope this helps.