One other hint to make things go more smoothly: Defrag the partitions
involved before resizing. Do a CHKDSK after the defrag to be sure that the
file system is 100% OK, else use CHKDSK with the /F or the /R option to fix
the file system. Backup data, just in case, then resize. A reboot after
changing partition structures is nearly always required.
Finally, if you change the boot partition, it is possible that its volume ID
may change. That is no big deal, except that it is one of the many things
that XP activation examines (retail XP, not pre-installed by PC maker). If
you have made several other changes to hardware (or BIOS) within a 120 day
period, this could be the straw that provokes a re-activation. However,
that is no big deal, but it could be a surprise.