Where's mah space?

I've been using my desktop (Windows XP) for about 3 1/2 years; I got it from a renter place, so it's really about 4 years old.

The computer runs well enough, but I'm constantly running short of space (anime and porn addiction). I look at the statistics, and the computer can hold about 53 GB. I'm using about 25 GB, counting everything in My Documents and on my desktop (everything I download and make goes in one or the other). I should have about 25 GB of free space, but I only have 10 GB free.

I'm no computer whiz, of course. I just want some answers as to what is taking up all my space and what I can do to get it back.

>>1
Other stuff such as Windows and all its malware take space. If you use Internet Explorer, chances are that you have many MB in malware you got from it.