The same thing happened to me aways back.. I re-installed XP.. fortunately,
I keep all data files on a separate hard drive, so didn't lose anything..
try doing a repair installation with your XP CD, assuming that it is not an
OEM or recovery version.. if it is one of these two, consider placing your
drive into another computer as a slave device and copy anything that you
need to keep onto a CD before re-installing the operating system..