Did this occur before installing SP2 ? I would advise you try uninstalling Windows XP SP2. If you can't get into windows via safe mode to uninstall it, then you'll have to do it in the Windows XP Recovery Console. Methods are here:
I had exactly the same problem on my current machine when I installed SP2, and it only appeared several days after installing SP2. With analysis, I discovered that I had an ntoskrnl.exe file corruption of some sort, and found out that the problem was due to the use of a Boot Screen Editing Program. I attempted to restore the ntoskrnl.exe file with the default file, but no luck. The file in use was the SP1 version instead of the SP2 version. Turned out that the boot screen editing program locked onto the SP1 version. So in the end I had to uninstall SP2 in the Recovery Console.
Your issue may not necessarily be the same, however uninstalling SP2 would be a wise step, and would most probably resolve the issue. It is likely that there is some kind of file or driver which is not compatible with SP2.