|Formating will destroy all data on your computer, that means you will have absolutly nothing left, no Vista, no programs, no files and the computer won't be usable untill everything is reinstalled.|
That means you would need a Vista instalation disk and drivers for all your hardware as well as any programs you have installed, or a restore disk that came with your computer.
It may have a restore partition on the drive that you can make a set of restore disks from.
You can uninstall programs that you don't use, backup files that you created, documents, photos, audio and video files to dvd to make room if you need to make room.
You should always backup anyway.