What happened is the boot files for Win 7 were put on the active XP partition.
Can you tell if the Win 7 partition is marked active, or can you mark it active in Disk Management?
When partitions disappear, they probably no longer have drive letters. You can add those by right clicking and add. Right clicking is also how you set it to active. You can only set one partition at a time per drive to active.
You can use disk management from either XP or Win 7.
Once you have the Win 7 partition marked as active, disconnect the PATA drive, boot to the Win 7 install DVD (or repair CD) and go to the second window and look for the repair option on the lower left. Choose that and run a startup repair 2 or 3 times.
So you will know, the system will boot to the first active partition on the hard drive listed first in the bios under drive order (or something similar).
If something doesn't work correctly, reconnect the PATA drive and during boot go into the bios and set that drive as first. You will then be able to boot back to the PATA drive.