You'll need to recover the System Active boot files into Windows 7 to start it up and then add XP.
First mark Windows 7 partition Active: Partition - Mark as Active
Now boot the Windows 7 DVD Repair console, accept any offered Repair. When it won't start, boot back in, click through to Recovery Tools list to run Startup Repair up to 3 separate times with reboots until Windows 7 starts: Startup Repair - Run 3 Separate Times
When Windows 7 starts install EasyBCD 2.0, add XP, accept offered boot files, Save, Restart.
If this fail, I would use free Partition Wizard bootable CD to rightclick>Wipe the Windows 7 partition before trying install again. Residual bad code may have caused the installer to not configure the Dual Boot as it is supposed to do.
It is much easier BTW to use XP virtual mode within Windows 7, or Dual Boot on separate HD's booted via the BIOS Boot Order and Boot Menu and not Windows. http://www.vmware.com/products/player/