I would strongly suggest a second harddrive in a USB enclosure with the same or bigger size than the main one you are using right now. You can use it as a sandbox - set the jumper to master and partition it the way you like using Disk Management (compmgmt.msc) or your new Easeus software - whatever you like.
Then install XP on the new small partition on the external - that partition must be big enough to hold the OS and the pagefile (1.5 to 2x base memory + OS Size) times two is a good rule of thumb. The second and third partition can be equal sizes depending on how large you want the Linux partition to be. In any case non-primary partitions can be resized.
After you have formatted the partitions and installed the OS on the first(XP) and third(Linux) partitions you can reboot from the external drive (or simply swap out disks placing the external in the system as master and the current master as the external - both jumpers must be set to master or the external is not recognised) and create a user on the new XP installation, pointing the data store to the second new partition. How-to: http://www.tech-recipes.com/rx/1409/move-your-documents-and-settings-username-profile-off-of-the-c-drive/
You could even reconsider making a fourth partition where you store all your non-OS programs - up to you.
In any case after you have installed all the additional software you need (some no doubt from the original disk) and you have copied over your data from the old disk to the new disk location you can start testing your "sandbox".
When satisfied with your XP-setup goto work on Linux by booting from the third partition. Make a nice VM which allows access to your new data store for example. If you are all done you can choose to keep the complete configuration as is or swap out the drives again and go back to the old situation. In any case everything starts with getting a good external disk in a USB-enclosure.
Bottom line is you can fiddle all you want and go back instantly if you wind up in an undesirable situation. Make sure you do your backups on removable media, not some separate partition on the same medium to avoid a single point of failure.
Work ESD-safe when handling, swapping and storing harddrives. http://en.wikipedia.org/wiki/Electrostatic_discharge