Are you able to do a control - alt - delete keystroke combination?
It's the same one used in earlier versions of windows to reboot or to
at least get a reboot options windows. If you do it in XP, it brings
up Taskmanager.
When you get in there, look under the Applications tab and see what
apps are running. One of them might be frozen (not responding). If so,
highlight it and hit the 'End Task' button. You might have to repeat
this several times. Make a note of the application doing this. You
might need new drivers for it, or to run it in Compatibility Mode, if
it's an older app. Right click it's name in File Manager (Exlporer)
and select Compatibility tab. Follow instructions there.
If you can't control - alt - delete into Task Manager, you've got
deeper problems. It could be a virus, or a conflict somewhere else.
Or, XP might have gotten contaminated somehow, requiring a re-install.
If you upgraded directly from ME, that could be a problem. I feel it's
better to do a clean install of XP. I have Xp and Win98SE on separate
partitions on the same disk. I'm using FAT32 and they co-exist well. I
can access the files of either from the other.
If you have to re-install at any time, do it only as a last ditch
effort. Too many people re-install and even reformat as a general
solution. There are ways to re-install without losing all your
installed program registry links. You have to backup the registry but
you should be doing that anyway. Keep a spare copy of it somewhere for
such events. Also, find out how system restore works.