Could you state the models of Inspiron please? They might be different?
When you say "stops after a few seconds", when exactly does this happen? On the XP/Vista boot screen? Does it blue screen and reboot, or just freeze?
Also, on the DIMM that you transplated into the machine you're keeping, can you please tell me what markings it has on it? i.e. speed such as PC3200 / DDR400 etc? Can you please post the same for the sticks that are in the machine already.
Thanks
Jamie