got a problem with my hard disk on my laptop

over night ive gone from having 17gig (the hd holds 37gig) of space left to having 1gig left.

nobodys touched it or anything and ive backed up everything i had on it and removed all my programs apart from the core components of windows but i still only have 5gig left now.

ive gone through everything and cant find why or what has taken up all the space

somehow 12gig has been taken up by fuck knows what....

doing a system restore to a day last week or even any day doesnt work so can anyone help or tell me how to go back to the factory settings?

ive done it before on my desktop pc but cant remember how to do it.... i think its f8 or f11 while the computer loads up????

cheers guys!!!