Try hitting the F8 key just before POST completes, and just before the OS starts to load...which will bring you to the Windows Advanced Options menu screen. Choose booting to the "Last Known Good Configuration" (use the arrow keys to select Last Known Good Configuration...your most recent settings that worked), and then press ENTER.
Last Known Good is controlled by a Registry Key "ReportBootOK". It's primary use is for rolling back a device driver update, and works independently of System Restore. It appears in the "Advanced Options" menu as "Last Known Good Configuration."
When XP completes a boot sequence, it updates the configuration data in the C:\Windows\LastGood folder. If you make changes during the current session that won't subsequently boot, you can use the LastKnown to rollback to that verified configuration.
You could also try running the System Restore tool from the Safe Mode command prompt. Boot to the Advanced Options menu as before, except this time select to boot to Safe Mode with a command prompt | Log on to your computer with an administrator account, or with an account that has administrator credentials | Type the following command at the command prompt, and then press ENTER:
%systemroot%\system32\restore\rstrui.exe
...then just follow the instructions that appear on the screen.