1.
Current Status. It sounds like you are able to boot into Windows XP. Perhaps you could clarify your current status.
a. Are you able to successfully boot into Windows XP? With or without blue screen?
(1) Using "Normal" mode?
(2) Using "Safe Mode"?
b. What blue screen Stop Code are you "now" getting and when does it occur?
(1) At what point in the startup process?
(2) How do you get past this "fatal" error?
c. What service pack level is your Windows XP at?
2.
Chkdsk.
a. Which "chkdsk" did you run (from hard drive operating system or boot CD)?
b. Which command line did you enter (e.g. "chkdsk /r")?
c. Did you execute "chkdsk" against every logical disk drive letter (that's using NTFS file system)?
3.
BlueScreenView. Download, unzip, and run
BlueScreenView. Are all the blue screens consistent or do they differ... Look for anything interesting...
4.
Event Viewer. Open up the Microsoft event viewer and look at the logs for errors and/or warnings that correlate with the times of your blue screens. To access: Start -> Run -> type "eventvwr.msc" without quotes and hit <Enter> key.
While you're in the "event viewer", open the "Application" log, sort on the "Source" column, and look at the "WinLogon" entries to see what errors if any "chkdsk" reported.
5.
Steps Taken. Hold off on this a little while longer, but I doubt you've tried everything. Look for things that must integrate deep within file system to operate...
a. Have you uninstalled AntiVirus program?
b. Have you uninstalled other AntiMalware/AntiSpyware software?
c. Have you uninstalled third party firewalls?
d. Have you uninstalled backup utilities?