I think it would be better to install OS X onto a separate harddisk and install the boot files there.
Then you can add a boot.ini entry in the [operating systems] section to chainload OS X (copy chain0 to your xp partition as well):
C:\chain0="Mac OS X"
If you format your OS X disk as MBR then you're done, if you choose GPT then you either create a small msdos filesystem on that disk, in this case Disk Utility will sync your GUID partitions to the MBR area as well (up to 3 partitions). Or use a gptsync tool to do this task.