For now, remove the XP drive; just disconnect it from your computer, and let's focus on getting Windows 7 working right. Then, we'll add XP into the mix.
Make sure the Windows 7 drive is the #1 hard drive, in the BIOS.
Now is there a 100MB "system reserved" partition on the Windows 7 drive? If so, follow these directions to remove it: System Reserved Partition - Delete
You can also try writing a new boot sector to the drive. From the command prompt, enter the following:
Make note of the drive letter of your main Windows drive. It should be C, after you delete the system reserved partition, but just make sure. If it's different, substitute that for C, below:
exit //that just exits Diskpart
bootsect /nt60 C:
If that doesn't work, run a startup repair from the DVD, and hopefully that will do the trick.