GUID partition table (GPT) or map is a set of instructions at the very begining of a storage drive to tell the hardware what partitions and formats are where on the drive.
A Intel Mac now requires a GPT to boot OS X as it uses EFI (Extensible Firmware Interface) which is a software firmware in a hidden EFI partition on the boot drive designed originally for copy protection by Intel. EFI loads into memory before OS X does, which can be seen if you have verbose mode activated upon boot time.
Setting up the boot drive with a GUID Partiton table WILL require backing up of all data off the machine and a complete erasure of ALL partitions on the drive, which includes ALL data, programs, operating systems and files not backed up off the machine previously.
Since a partition map is basically road directions, when it's destroyed so does go the partitions.
Also since you will be fresh installing 10.6, your free iLife won't tag along,, however you can erase/install 10.5 first then upgrade to 10.6 (no BootCamp) and that shoudl work.
Unfortunatly Mac's only now support Windows 7 in Bootcamp, however Windows 7 Pro (and above) will run XP programs natively or via free virtual machine XP downloaded from Microsoft, however since it's really not native, 3D games etc likely won't run very well.
If your not familiar or willing to take a chance, then I suggest you have someone else upgrade that machine.
https://discussions.apple.com/community/notebooks/macbook_pro?view=documents
