Only you will know what personal files you can delete.Not sure what you mean by user names? Do other people use your PC? If so then they would need to back up there stuff.
But I would suggest if as you say you have backed up everything then how about a fresh install of Windows 7 which will give you a lean fast PC. But rather than install Windows on what looks like a rather small (250GB)hard-drive why not buy a 1TB hard-drive, install Windows on it and use the 25GB HDD as a internal backup?