You have two possibilities...
1. they may have a restore function key that should show up after you power up the machine. There's the common F2 for setup F12 for boot options.
2. You can acquire the install cd/dvd from Dell (May cost you, tho not sure how much.)
If you computer has a Vista COA sticker on it, you can install it from another vista cd/dvd and enter your COA key when it asks for it.
There may be other ways of doing this but these are the first that come to mind right now.
cayodread
February 2009