by starting the computer with switch and pressing F8 key every second.
This should by pass main Windows boot up.
You should see a menu of choices including "safe mode".
Use Up or Down Arrow keys to choose safe mode & press "enter" key.

IF this works ..try using System Restore to go back to when you KNOW it worked OK. OR (if system restore fails) try (in safe mode)
Checkdisk/ Repair as it may repair XP system files. Here's How:

Disk/repair (Check Disk) (like SFC)
Left click on My Computer(open)
Right click on "C" or your OS drive if another letter.
Left click Properties and then click Tools Tab.
Left click on "Error Checking"> Check Now.
Left click to enter check mark in BOTH boxes offered.
Left click on "Start".
Computer will have to reboot to begin repairs.
Just leave alone (you're locked out anyway) 'til process finished.
In regular mode will take about 1 Hour (more/less)
In Safe Mode about 2 hours.
If computer normal after process complete you might want to create a new
restore point and Label it POST ERROR REPAIR.
This is based on using an onboard copy of sys
files if copy is ok.
If copy is corrupted, it may tell you to insert XP or SP2(if patch applied) disk or to indicate location of SP2 info to get new,
clean copy inserted. Good to have disk at hand. Good Luck!! Happy