Any operation on partition has the chance of it messing up so it's a
wise move to make a backup of your data and perhaps a drive image so
that it can be restored if it all goes horribly wrong. FWIW for a
simple single partition drive such as WinXP probably has it's rare for
things to go badly - it's more convoluted partition schemes that Vista
introduced that cause the most headaches.

Well worth running CHKDSK /R on the drive first and also defragmenting
Windows.