all you need do is to delete the partition with xp on and split the hdd up using the xp pro disc. put the disc in and boot from cd. press enter at the first prompt, then esc at the secound. when your installation appears onscreen press d to delete, you may have to press enter then " L" to complete it.
now press "C" to create a partition, press the backspace key to clear the size box then key in the size of the partition you want to create. remember that to create a 10gb partition key in 10000. you'll need at least 5gb for xp. press enter. you're taken back to the setup screen and you'll see the partition you've just created and some freespace. highlight the free space and press "C" again and create another partition.
highlight the first partition and press enter to install xp on it. once installed put your 2000 disc in and boot from it. set 2000 up on the second partition. you will have 2 bootable partitions. as they will both use ntfs you will be able to access 2000 from your xp and vice versa. no need to pay out for disk partitioning software. you could always take a look online for a free partitioner if you feel more comfortable with one. try http://www.majorgeeks.com