2 Computers, switched Harddrives, now 1 won't boot...

Hey guys, I decided to give one of my nice computers to my brother so we switched hard drives. Now his computer with my hard drives works fine, but my computer with his hard drives gets to the windows xp booting logo and then restarts, even when trying to get to safe mode. What did I do wrong?
(Before we hard drives swapped I tried my graphics card and ram in his computer, but it didn't work, so I put it back in mine. Thats the only changes i've made to either computer and I've rechecked both to enable they are properly seated, which they seem to be.
Any ideas why it won't boot? Oh yeah one is an AMD based machine and one is Intel, but that shouln't make a difference right, seeing as one combination works.

 

Are these PCs identicle? Did you reload windows?
If these systems are different you'll need to reload windows. One because it is loading drivers, ect for a different MOBO and possibly causing the crash and
two - XP only allows a few hardware changes before re-activation...

 

Thanks for the quick reply!
No they arn't identical, one is an AMD machine, the other is an Intel. Thing is the hard drives from the AMD work in the Intel (i'm using it now, no porblems) but the crash happens on the AMD machine with the harddrives from the Intel. What you said makes sense though, i've had that problem before when trying to use one computer's hard drive in another computer. Is reinstalling windows the only option?