Speed-up computer (Urgent)

CCleaner as several people stated above is amazing. I use it weekly on my MacBook Air and it keeps everything in ship shape. If you happen to be running OS X, which it doubtful since you are having speed issues I suggest also installing Monolingual and giving it a go. I used it to remove all language files that weren't English, and it saved me almost 5GB of space and was a noticeable speed increase.

Get solid state drive - you can get them with kits as well that makes transferring your data over easy.
That will speed your computer up far more than any cleanup will.