You can save space by right-clicking on the C Drive in My Computer and doing Disk Cleanup. Then choose to remove everything it pops up (like old installations). You can empty your Temp Folder, and some cached installation files as well (such as C:\Nvidia). You can also change System Restore settings to use less disk space.
How big is your hard drive? I have no problems with Windows 7 on my 80GB old laptop HDD. But I don't install much. There's no reason to have less than a 500GB hard drive on a newer computer.
Just uninstall some programs you don't need (Add/Remove Programs), delete videos you don't watch, and if you have another hard drive, do Custom Install options when you add new programs so that they don't install on C. With a small HDD, you have to do a lot of housekeeping to avoid running out of space. That's a major reason I've got several terabytes at home.