Laptop freezes, sometimes restarts

Right i'm looking into fixing a laptop for a friend, and i'm having problems.

I'm studying computer repair atm, but was looking for some input incase i'm missing anything.

When he first gave it me, it wouldn't boot Vista, but he wanted it removing and XP being put on, so that was no problem.
The HDD was giving SMART errors too, so we got it replaced.
When i've tried to install XP, the machine either freezes, doesn't boot, or just locks up and freezes after a random period of time.
I've tried swapping out the RAM with different sticks, and no change.

*When the machine *doesn't boot*, the power switches light up, whether it be mains or battery. The back light doesn't come on, but the DVD drive does try to read the disc. Its as if the laptop is working behind the display, but its unresponsive to everything

*I've run Memtest and i've had no errors, it ran for a good 36 hours.

*Loading Ubuntu has some success, sometimes it'll just reboot the system when loading, sometimes it'll load and freeze.

*Often the freezing occurs before the option to load Windows/Boot from CD. Sometimes before POST, sometimes during.

So the main culprits i can think of would be:
*The Processor
*The Motherboard
*Built In Graphics Card

The machine itself is one i'm unfamiliar with before now, its an Iqon M66N.
CPU: Intel Core 2 Duo 1.66ghz
RAM: 1GB
Built in Graphics
Western Digital 160GB HDD

Another idea i've had would be that i've noticed the network card light is always lit, regardless of its activity. Even when it has no driver installed, its lit up. Could it be a faulty part which is causing these restarts? However this could be normal activity for the model.

Any help/suggestions would be welcome
Thanks guys!