As gaz already posted you need this. Dual Boot Installation with Windows 7 and XP
Go to Method Two in this tutorial and start at Step #2. You already did #1.
You will need to use the XP disc to boot back into XP and download Easy BCD Step #6.
Step #7 download .Net Framework 2 if it is not already installed. This is needed to open Easy BCD in Step #8.
Follow directions from there for the Easy BCD.
This will give you the choice of booting to Windows 7 or XP after the bios screen.
I have the same set up. So know from experience.