My windows directory is huge

I have always had problems with my windows directory's growing to a huge size. It started with windows 95 then 98 then 2000 and xp. Windows 7 however is absolutely huge. The windows folder itself with out program files is 21.4 gigs...add the program files and other windows files and it is a whopping 32 gigs. My question is why is it so large and what can I do about it. At the rate this is going I will be out of space in now time.