i turned my laptop on the other day and i get a blue screen

STOP c0000218 registry hive file
.....system32/config/software
not writable or whatever

i cant find my vista premium dvd so id like to do a clean install with my xp pro cd.
i cant get the dvd drive to work though i just keep getting thrown to the blue screen.

what should i do?

I'd suggest contacting the computer manufacturer for a set of replacement Vista disks - it'll be a lot easier than hunting down all the drivers that you'll need for XP.

That being said, there should be a key combination that you can press to get to the boot menu/BIOS on your laptop. Here's some of them:
Toshiba - F12
Gateway/eMachines - F10
Dell - F12
Sony - look for the prompt to boot from CD
HP/Compaq - Esc