SP2 Crash - Now can't boot

I am trying to evaluate Parallels to see if it will work for my situation.

I installed the free evaluation version - tried running my quickbooks installer and it said I needed service pack 2.
So I ran window installer and got an error halfway through to which it said it was un-installing sp2.
Now when I try to boot into windows XP I get to the desktop screen and then in BSOD on me and reboots - and keeps going.

I am new to Parallels. SO my question is, how do I re-install windows when I can't even get it to stop re-booting?
THEN, how do I successfully install SP2?

Macbook
2.2 Ghz / 4 Gb ram
OS 10.6.6
Installed Windows XP Home Edition