Boot from CD - How to Boot your Computer from a Bootable CD or DVD - Boot to CD
Make sure you are following the steps in link above to boot DVD.
Select Repair on second screen, see if it finds an installation to repair, accept repair. If this fails to start Windows 7, boot back in, click through to Recovery Tools list to run Startup Repair repeatedly to see if it will repair Dual Boot and/or Start Windows 7.
If not, you'll need to see that Windows 7 is marked active using a 3rd party bootable Disk manager like free Partition Wizard CD, or Diskpart from DVD Command Line, then run Startup Repair up to 3 separate times with reboots to write MBR to Windows 7 which can be done once it is marked Active.
Once Windows 7 is started - likely without Dual Boot menu - install EasyBCD 2.0 to add XP, accept boot files offered, let it autocomplete, Save, Restart.