Okay, what version of xp pro do you have?? If I remember right if you have an older version SP1 or before the OS doesn't recognize any HD over 130 gb's. If you update your OS to SP2/3 than the OS will recognize the bigger HD, but you'll have to do a couple of things after the update.
Here is what you'll need to do:
* Go and click on the START button
* Go to Administrative Tools
* Computer Management
* Disk Management
* And then format the unformatted portion of your HD. This will give you the 2nd partition, which will be what is left on the rest of your HD.