Running out of hard drive space?

So I have a 128 GB SSD (C: drive) which I installed Windows 7 onto, and a 1 TB HD (E: drive) that I use for everything else. When I first built this PC, I had a little over half the space left on the SSD, but its slowly been filling up with god knows what, now I only have 18.5 GB of space left. I don't download anything to my SSD, and I don't install anything to my SSD. I download and install everything onto my 1TB HD. Does anyone know why my SSD is filling up so quickly, and how to clear it up? I've thought about just reformatting it, but a lot of Steam games have their save files on the C drive, so they end up on my SSD, and some of them have their save folders waaayy deep in the dark corners of the C drive that even if I were to back them up, I couldn't be sure I didn't miss any. Any advice?

Updates.

Im fairly certain you've got other stuff other than windows on there. Video card drivers/software? MS Office? Another browser? Some utilities? Adobe Reader or Flash? File archiver (WinZip/WinRAR)? Itunes or another music player?

All this stuff slowly adds up. And every time theres an update, they always add functionality, and everything increase by just a little bit in size, just a bit by a bit. And yeah, slowly, it'll creep up and add up in size. Its one of the reasons i like to reformat once every year or every other year. There isn't likely any *one* single thing that is ballooning in size that you can remove to get back that space.

Even if you don't install programs on the C drive, a lot of their files end up there anyway. Also game saves, data from programs, other random stuff..
What you can do to help a bit is go to your user folder, and move those folders to another drive (My Documents, Downloads, My Pictures, etc..). You can do that by right clicking on each one -> Properties -> Location -> Move. That may or may not help, depending on how much stuff you have in there.

Unfortunately that won't move the AppData folder, which can get quite big on its own. You can instead move the entire Users folder to another drive and solve the whole problem, but that requires a fresh installation of Windows. And I don't really remember how it was done. Google should help you with that.

A few things to check

1) download TreeSize FREE, it will show you where the big data is

2) Delete old UPDATE install files (google how to do it)

3) Lower the SYSTEM RESTORE point percentages, and delete old restore points (google how to do these)