Hey, I used to use Windows 3.1 like it was the best thing ever!
Wasn't until I was 11 that I finally experienced Windows 9x and not until 13 that I had Windows XP. You kids are spoiled
Anyway, the problem is that there is a major hardware clash. The computer you installed XP on had different hardware and required different drivers to boot up. The computer you put the hard drive in (the original computer) has different hardware and needs different drivers and had a clash when the different drivers were trying to load.
To fix this, all you need to do is reinstall XP while the hard drive is in the computer you want to be able to use it on. You can always take the hard drive out and get files from it on another computer, but it won't boot up from any computer that has different hardware (or different drivers needed).