AT7-MAX 2 won't power on

I recently built a computer for my flatmate. I've built a few
computers before, and don't think I've made any particular oversights.
Before I describe the failure, here's the specs:

ABIT AT7-MAX 2 motherboard
Athlon 2600+ XP
2x 512Mb PC333 RAM
ABIT GeForce4 Siluro FX5800 with 128Mb RAM
Plus a harddrive in IDE1, and a DVD reader in IDE2

After the first attempt, everything seemed to go okay. I installed
Windows XP and all seemed good. Then we went out to get some stuff
while leaving the computer off, returned, moved it into another room
and switched it on. Fans spun up, but there were no beeps to my
recollection, and the hard-drive lights didn't go. Then it cut out and
shut-down. The orange light in the corner of the board stayed on, but
it didn't respond to the power button. On the advice of the shop we
bought the parts from (www.scan.co.uk), we stripped it down till we
had the motherboard sitting in its plastic box, connected to the power
supply, its speaker and the power button. The light in the corner
would come on, and the northbridge fan would twitch when we plugged in
the power, but it wouldn't start up and give error beeps. We returned
the board and they (eventually) checked it, saying that it had a fault
in the USB controller. So they replaced it.

Now, we rebuilt it. Things seemed okay. We installed some audio and
graphics drivers, and GTA3:Vice City. Half-way through him playing it,
the machine cuts out and again refuses to acknowledge its power
button. I've not yet done the basic tests, but it looks like it might
be the same problem.

Soooo... what to do? I was a bit worried that a 300W power supply
might be insufficient for the processor and monstrous graphics card
(the thing has its own thermal-exhaust port and power-input). Is that
a risk? Would it cause these symptoms?

Are there known problems with these components? I thought that buying
ABIT for motherboard and graphics card would be a good idea. Now I'm
not so sure. System memory is a bit close to one of the graphics card
heat-sinks for my liking.

I'm not relishing the thought of returning this one too. I can't think
of anything I might have done when putting the parts together to cause
this. All risers were carefully installed, and I made doubly sure
there were none unaccounted for to accidentally short the motherboard
from the back. Do I need to return this to the shop? Should I try to
get a different motherboard next time, if this one is unsalvageable?

Regards,
Weeble.