go here.
http://download.cnet.com/ccleaner/Download and install the software (just 3 mb
)
after you've installed, close ALL YOUR INTERNET BROWSERS (like google chrome or firefox)
now, open your software and click "analyze" this will figure out how much useless files are stored on your computer.
and now click "clean" this will remove all the useless files.
Just make sure bookmark all the sites that you need, 'cause it will delete all your web history, cache and other data.