Well one way is to get a live CD with gparted on it. I like Parted Magic but you could even use your Linux Mint CD. Use Gparted to delete all the linux partitions and then extend XP back to its original or leave that part of the drive unallocated. Its up to you. Now you will run into a problem with your boot loader. You will need to stick in your XP CD and do a repair. Once it brings up the command prompt do the fixmbr command and that will restore your XP bootloader. Now you can reboot and get back into XP without any trouble. If you are just going to reinstall mint you don't need to do that step just start the installation. If your looking for some good suggestion on how to partition your drive I would look at this excellent post by Fred http://linuxmint.com/forum/viewtopic.php?f=90&t=11872
. In fact I would read that a bit before you do much work with partitioning at all.
I hope I answered your question and good luck.