Hi Bob21. Is this a problem that has just occurred on an old installation, or is it a new Windows XP installation?
Have you tried booting into Safe Mode by holding down the F8 key during boot-up?
Have you tried leaving the computer to boot up, I am having a similar problem with an XP installation that would eventually boot after about 20 minutes. I have found a work-around for that by disabling "Shell Hardware Detect" in "Services". I still haven't found the reason for this behaviour.