Two possibilities come to mind, based upon the utter sparsity of
information provided:
1) Boot into VGA Mode or Safe Mode and then set the display
(resolution, refresh rate, and/or number of colors) to something that
your monitor can handle. Consult its manual to determine acceptable
settings. Reboot one more time, into Normal Mode.
2) Download and install the most recent WinXP-specific video drivers, as
provided by your video adapter's manufacturer's web site.