Does keyboard work in BIOS, because the best way to Dual Boot with separate HD's is booting via BIOS?
Set Windows 7 HD to boot first in BIOS setup (after DVD drive), then when you want to boot XP tap the Asus F8 BIOS Boot Menu key at bootup to select the XP HD from list.
To do this both HD's need to be marked System in Disk Mgmt but it's likely only XP currently is if it was installed first - check now and post back a screenshot if you want using SNipping Tool in Start Menu.
To write the System boot files to Windows 7, mark it Active, power down to unplug XP HD, set Windows 7 HD to boot first in BIOS setup, reboot into Windows 7 DVD Repair console or System Repair Disk
to run Startup Repair - Run 3 Separate Times
until Windows 7 starts on its own.
Then plug back in XP HD, boot it when needed using F8 key at boot.