First of all I advise you to back-up your files any time you are about to repair your operating system especially if you don’t consider yourself an advanced user.

Also you need to know that if perform repair installation for Windows Vista few items will be reset to default - Windows mail, Device drivers, the computer sound event scheme and a few more.

Please read carefully the following article - How to perform a repair installation for Windows Vista, make your back-ups, follow the instructions, and be patient as it might take up to four hours depending on your computer hardware.