Windows XP's Boot.ini can not boot Windows 7... you need to Edit Windows 7's bootloader to include Windows XP.. You can do that easily with EasyBCD Community Edition installed on Windows 7.
If you had installed Windows 7 with Windows XP already in the Hard Drive, the Win 7 installation would have automatically included Windows XP in the bootloader, so I figure you installed XP after Win 7(?)... So, to boot into Windows 7 use the BIOS boot selector... the BIOS post should say which key you need to press to make the boot selector display on the screen. And if you can't find which key to use, go to the computer or motherboard manufacturer's website and dowload the user manual to find that information in it. Or use the Windows 7 installation DVD to fix the bootloader.
I've always had trouble finding a EasyBCD guide or tutorial that's easy enough for anyone to understand it but you don't really need one, as the program is quite easy.. I added some EasyBCD screenshots you can use as a guide and also added a link for a text user guide just in case you need to find additional information on how to edit the Windows 7 bootloader... I find that guide more complicated than using the program but it's the only one I could find. Maybe a video can guide you better.
Fixing the Windows Bootloader via the setup DVD
https://neosmart.net/wiki/recovering-windows-bootloader/
EasyBCD Community Edition 2.2.0.182
http://www.softpedia.es/programa-EasyBCD-45820.html
Click on the EasyBCD image to see some screebshots and use them as a guide.
https://neosmart.net/EasyBCD/
User Guide
https://neosmart.net/wiki/
How to: Dual-Boot Windows XP and Windows 7 (7 installed first)
http://www.youtube.com/watch?v=WmCZvrctECE
Install Windows XP in Dual Boot with Pre-Installed Windows 7 by Britec
http://www.youtube.com/watch?v=mWRewE5GIcE