Here is a good guide to dualbooting XP and 7. Basically, when you installed XP, it wiped out your Windows 7 bootloader and you need to repair it. Here are the relevant sections:
7. In XP, download and install .Net Framework 2.0 (32-bit) version or .Net Framework 2.0 (64-bit) version first, and EasyBCD (free version) for your 32-bit (x86) or 64-bit (x86) XP.
NOTE: In XP, Net Framework is required to be installed to run EasyBCD. After you click on Register (at bottom of link) for the free EasyBCD, registration is not required to download it.
8. Run EasyBCD (free version).
NOTE: This is required to repair the Windows 7 boot file and add XP to the Windows Boot Manager list.
9. On the left side of EasyBCD, click on the Add New Entry button.
A) In the top section under Operating Systems, click on the Windows tab.
B) To the right of Type, select Windows NT/2k/XP/2k3 from the drop down menu.
WARNING: Be sure to leave the Automatically detect correct drive box checked.
C) To the right of Name, you can leave the default Microsoft Windows XP as the name to be displayed in the Windows Boot Manager, or you can type whatever name you would like to have instead.
D) Click on the Add Entry button.
10. On the left side of EasyBCD, click on the BCD Deployment button, select (dot) the Install the Windows Vista/7 bootloader to the MBR option, and click on the Write MBR button.
11. Close EasyBCD.
12. Restart the computer to have the option to boot from Microsoft Windows XP or Windows 7.