Unplug the XP HD, plug its data cable into Win7 HD, make sure it remains set to boot first in BIOS setup.
Now mark Active the Win7 partition: Partition - Mark as Active (Method Two)
. If it won't mark Active it might be a Logical partition (which is why we needed to see the Disk Mgmt screenshot first). Boot into free Partition Wizard
bootable CD, post back a camera snap of drive map with listings, or just convert to Primary the Logical Win7 partition so it can be then marked Active.
Now from the DVD run Startup Repair - Run 3 Separate Times
until Win7 starts and holds the System Active boot flags in Disk Management.
You can now plug back in the XP HD if it is not formatted to access it's data from Win7 explorer, or boot it if needed by tapping the Dell F12 one-time BIOS Boot Menu key at boot to choose XP HD to boot.