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The problem is that you detached the XP drive when you installed 7. As a result 7 didn't find a boot loader and created it's own on the 7 drive. You may be able to fix it with 'Easy BCD'. You can find a link here: FREE Great Programs for Windows 7
If you boot into 7 with both drives attached, you should be able to add XP to the boot menu so you get the choices during boot.
Hope this helps.