Hi Rob, welcome to NST.
did this fail on SP1 or SP2 ? through Windows Update or downloaded manually ?
SP1 had an issue whereby the WUD install appeared
to hang at 99% complete but was actually continuing invisibly in the background to update the HDD.
Users who lost patience and hit reset before it was really finished (I waited 30 minutes after the lights had stopped blinking) crippled the install.
SP2 went on cleanly without issue on my PC this morning (through WUD).
If you let WUD install the SP, it will have taken a system restore point before starting, and you should be able to boot your Vista DVD (if you have one) and select system restore after "repair your computer"
If you don't have a bootable Vista DVD you can download a recovery CD ISO image
from this site which will do the same job.
If you downloaded and installed the SP yourself without setting a restore point, your only option is a factory reset or a Vista reinstall using the Dell recovery partition or bootable media you created when you first got the PC.