hi guys, i'm trying to install Hardy on my Desktop. When i choose the installation choice and it starts loading the system, after a while it reboots!!! It doesn't even reach the desktop!

I tried another CDs, i tried also xubuntu but i got the same result. I also tried to boot as Live, but it also reboots while it loads the system.

what should i do??

PC spec:
CPU: P4 3.2GHz.
RAM: 512MB DDRI 400mhz.
Mobo: asus ....

Thanks.