Had any luck getting that hard drive out or borrowing a copy of XP?
Just looking back at what you said, you only took screws out of one side, I'll bet there are screws on the other side so you will have to figure out how to gain access to it.
If you can check for that i386 folder on the other PC and check for the version of XP you might be able to create a boot disc that will get to the Command Prompt, you can then type in a command that will repair the Master Boot Record and hopefully get it to boot into Windows again.
Can you tell me what version of XP is written on your licence sticker. Home, Professional, etc. Have you always had Windows update turned on, would you know if it is updated to SP3?