Well if you run the disk clean up tool that comes with W7, before you delete anything, it will tell you the size of numerous and various categories of files/folders it can delete and the amount of space it can recover for you.
It's possible you have a big windows.old file with backed up data created during the W7 installation. I believe the disk clean up tool will allow you to delete that file, among others, if you want.
It is found in: Start, all programs, accessories, system tools, disk cleanup
Also, if you want to recover more disk space, you can go into system restore and there is a slider there that will allow you to adjust the amount of disk space you want to allow system restore to use. I have mine set for 10 GB, but you could certainly get away with letting it use much less space. Probably 3 to 5 GB would be adequate for most users.
