Use Mbr Fix to fix boot record first. I used it on my Vista 64 system, worked great. Run from cmd in Windows.
Download here:
Download
Instructions here:
MBR Fix - Fix MBR problems
Now Grub is no longer on your boot record. Your PC will boot straight to Windows.
Vista has it's own partition manager built in, but you're running XP so use Partition Magic or Acronis to delete/expand partition.