Have you tried system restore? or last known good config? have you performed any changes lately? if all else fails then you could do a repair installation...
Start the operating system from the CD-ROM,
When the computer starts from the CD, the system checks your hardware
and then prompts you to select one of the following options:
To set up Windows XP now, press ENTER.
To repair a Windows XP installation using Recovery Console, press R.
To quit Setup without installing Windows XP, press F3.
Press F8 to accept the Licensing Agreement.
A box lists your current Windows XP installation, and then the system
prompts you to select one of the following options:
To repair the selected Windows XP installation, press R.
To continue installing a fresh copy of Windows XP without repairing,
Press R to start the automatic repair process. After repairing Windows
you may need to reactivate Windows XP, if you changed some hardware.
Back up and save the Wpa.dbl file after successful activation and then
restore it after the repair process is finished and after you log on to