Hello. Sorry to barge in here, but I'd like to give you a more relevent answer to your question. Please don't consider reformatting your hard drive. You didn't indicate if you were running a PC, but I am going to assume you have a Windows system. In order to clean up all your temporary files, including downloads, cookies, and broswer history:
In Internet Explorer, click on Tools on the top menu
Click on Internet Options
In the section of that window labeled "Browsing history", click the "Delete" button
That's it - all your temporary files, etc. will be cleaned up. Another thing you might want to check is your Recycle Bin - open that by double-clicking the icon, and if there are many files there you can get rid of them and free the space by clicking "File" and then "Empty Recycle Bin". There are lots of other things that could slow down your system, but this will be a good start.