My understanding of your situation is that you have installed Win 7 in a different drive than XP and you want to dual boot but when you boot the computer you get errors and no boot to either operating system? Is that correct?
Sounds to me that you need a boot manager installed that will correctly locate both operating systems and rewrite the boot sector of the main boot drive to properly point to the operating systems and setup the boot menu.
I had an issue setting up dual boot on a raid drive, had the partition order incorrect and every time I booted I got windows cant find some critical file and halted.
I had to use a 3rd party software to go in and rearrange the partitions correctly and then booting and dual boot worked perfectly.
I have 3 partitions on that computer.
1: XP Pro
2: XP Pro cloned of partition one
3: backup partition
The problem I had with not booting was the order got rearranged during a over haul I did.
1: xp pro
2: backup partition
3: xp pro clone
I just went in and rearranged as I have written in the first ordered list above and everything worked right again.
That is what it is sounding to me that the boot sector of the drive doesn't know where one or both of the operating systems is located at.
Go check out the offerings at http://www.paragon-software.com/products/home/
I use one of their disk management suites and it handles everything I needed including automatically finding all the operating systems that are bootable on all drives installed and adding to their boot manager.
I hope this helped.