For dual boots, you will need to have them on different partitions or different drives. If you have both Windows on the same partition you will have conflicting files installed (eg. same Program Files folder, etc.) which would confuse both operating systems.
After successfully completing a dual boot install of Win2003 Server and WinXP Pro (WinXP Pro being the second to be installed). A possibility to the installation failure is a hardware issue. Likely causes are scratched CD, bad multimedia drive, bad hard drive, or RAM chips gone bad.