As i said in comments, you should install 2003 in another partition and to do that follow the next instructions:
From within XP put a name on the partition you want to install ("2003", "server", etc) something you can remember. Also you may want to leave that partition empty (don't mind the System Volume Information folder).
Now, boot the 2003 install CD and follow the instructions to install, when prompted to select a drive/partition, you will see the partitions and their names, select the one you named previously. It will later ask you to format it of leave as it is (your choice here) and proceed with the install.
After the install is finished, during boot, you will get the boot menu showing your XP install and your new 2003 install. You can then change the default OS boot from within XP or 2003.
Note: I have done this several times without incidents (no data loss, etc)
Note 2: Be aware that after installing 2003, the 2003 system drive letter may or may not be C: (I've had mixed results) but the system is completely usable and i hadn't found any issues with it even when my 2003 was installed under the G: drive.