Skip that previous post, this is a better procedure....
Upgrading with the SP2 pack occasionally does that. The web is full of advice on SP2 installation problems, M$ has a good guide on it also. I think the easiest way is to follow this procedure until something works:
Restart your pc; press F8 several times while POST is running and before IDE detection completes.
On the Windows Advanced Options Menu, select Last known Good Configuration. If that works, fine.
It did not? Then restart, press F8; when the Windows Advanced Options Menu appears select Safe Mode and press Enter.
Select your installation if given the option, login using an administrator account, else as the administrator [default password for that is Enter].
You want System Restore, so press No in the popup window. Choose a recent restore point and see if your system starts in normal mode.
That did not work either? Then restart, this time press F11 while BIOS is running to enter the boot menu, select CDROM and insert your XP installation CD, and off you go. You want to do a Windows repair... You will be given the option to repair with Recovery Console, or to press Enter to begin Setup -press Enter. [You do NOT want the Recovery Console]. Your existing XP installation should be detected, you select it and opt to repair it; what follows is a little like a full installation, but your existing data files are NOT damaged.
When you get XP in normal mode try SP2 again.