It should be the same as you would for XP.

Probably a good idea to leave free space for Linux when you partition your harddrive for 2003. Some Linux install's can resize the partition for you though. Always make sure you have backed up all your important data.

Install Windows 2003 then install Linux, depending on the distro of Linux you are using it should pick up the Windows install and add it into the Grub/Lilo boot menu.

I hope I wasn't too vague.