Shoutbox

Could be overheating. Most modern systems are designed to reboot or power off if the temperature gets too hot.

Also if the CPU is in any way different, for example if it's an AMD 2700+ to a 3200+, then the FSB speed has increased, which means that you may need to upgrade to PC3200 RAM due to motherboard limitations (some motherboards run a 3200+ using 333MHz rated RAM fine...)

You can usually monitor temperatures from within the BIOS.

I wouldn't personally expect something on the motherboard to have "blown-up" if the system is getting to the loading screen, although I guess it's not impossible.