You can right click on the folders in Windows and check the properties of each folder. Then find the one with a large size and keep limiting it down to find where this is coming from. Or you can get a program that will show your drive in a pie chart and you just dig through that to find out where all the space went.
Okay, I ran windirstat and found that my "recordings" folder, where I keep all of my Fraps recordings, was taking up a total of 220 GB of space!
However, right now there's a total of maybe a dozen different recordings in the folder. All of these being around 4 minutes long and taking up around 3.5 GB each. And of course, they're only this size because I've been to lazy to compress them. I've recorded probably thousands but I just delete the one's I don't want.
So, how can this folder be taking up so much space?