I received a laptop that had XP Service Pack 2 on it. During the reboot after installing Service Pack 3, the laptop blue screened. I could not get in at all. A little elbow grease and some googling, and I was able to get back to where I was before the original upgrade. Here are the steps I took.
1. As I could not get past the blue screen, I had to use recovery console. Put an XP cd in the drive and boot the machine. (Of course, on my machine, I had change the boot order in BIOS.)
2. Press any key to boot from the CD.
3. When the "Welcome to Setup" message appears, press R to start the Recovery Console.
4. Select the Windows XP installation (I had only 1.)
5. Enter the admin password (a miracle that I had this.)
6. When you get the command prompt, type: cd $ntservicepackuninstall$\spunist and hit enter.
7. When you get a prompt back, type: batch spuninst.txt and hit enter.
(You will see error messages and files being copied, this is normal.)
8. Remove your cd.
9. When you get your prompt back, type: exit and hit enter
10. Restart the computer in safe mode.
11. Log in as administrator.
12. Either open Control Panel, or Start -> Run and type: appwiz.cpl
13. Click the check box to Show Updates.
14. Scroll and find Windows XP Service Pack 3...and click remove.
15. Click Finish to restart the computer after the removal process is finished.
Now, I have to re-apply Service Pack 3 to get this computer up to date. But, at least I'm beyond the blue screen.