You can't just swap hdd's across from one machine, unless both systems are identical in all harwdrae and bios configuration.
The reason the swap from eMachine to Dell worked, is because W7 has a larger driver database and can reconfigure itself far better than XP.
To continue using XP on the eMachine, you will need to install the XP motherboard chipset, video, network and sound drivers and pray it all works.
LOL, thanks much! I was pretty much sure that was the situation. Sadly I don't even think the prayers would help. I don't really like eMachines much and should have stuck with another. Thank you again for your help!