Hard drive space is slowly going

Hi i recently install vista on my laptop now it works fine and the only problem is my hard drive maximum size is 37 gb now it was at 30 gb left now its slowly going down. can anyone help me? thanks in advance.

Update

also when i goto play any media files its starts then gose straight to the end why is this? thanks in advance again.

[369 byte] By [SweptSquash] at [2007-12-24]