Enter the system BIOS (typically by pressing either F2 or DELETE during boot on a Dell) and ensure the first/primary boot device is the optical drive, not the hard drive, exit saving changes, then reboot with the first operating system DVD in the drive. It should either boot to the Windows Vista installer automatically or prompt you to press any key to do so. You may also try doing this with the hard drive disconnected. If it fails to boot to the operating system disc you may have a faulty DVD and have to order one (usually around $35) from Dell.

As to saving your data, you can do so without an external enclosure if you connect your hard drive internally to another computer. Boot to that computer's operating system on its hard drive and use it to browse your hard drive's contents, remembering to take ownership as necessary.

Best of luck.

P.S. In the interim, download Ubuntu and burn a LiveCD. (Instructions on their website.) It is a free, alternative operating system that runs from CD, enabling you to access the internet, type documents, etc. from your computer when Windows and/or your hard drive is unavailable.