Hi Donato, welcome to NST.
As you installed W7 to a separate HDD which is now "system", it should have left XP in its original single-boot condition.
You can check by putting the Xp HDD first in the BIOS (with a temporary override using F8 or whatever your BIOS uses to access the boot override dialogue before the power-up splash screen.)
If XP boots normally, you can just make it permanently first in the boot sequence and format the IDE drive.
To add to Terry's advice above, if you replaced XP's boot code on its own HDD with the W7 version, then this will require putting XP's boot code back. The easiest way to do this in your situation is to boot into XP from the W7 bootloader, and then (while in XP) disconnect the W7 HDD, start up EasyBCD 2.0 Beta, go to Manage Bootloader, select the "Uninstall the Vista bootloader" option, and click the Write MBR button. Then you should be able to boot into XP with the W7 HDD still disconnected. You should test it though first before formatting your W7 HDD, to avoid making things harder on yourself.