Crysis crashing after 5 mins (new system build)

Hi. Built a PC for my younger cousin and it runs absolutely fine. That is, apart from when he tries to run Crysis. It runs for around 5 minutes then locks up and a system restart is needed to get back into Windows.

It comprises:
Asus P5N-E SLI 650i Socket 775 PCI-E
OCZ 2GB Kit (2x1GB) DDR2 800MHz/PC2-6400
Point Of View 8800GT 512MB GDDR3 DVI HDMI PCI-E
Intel Pentium Dual Core E2180 2GHz Socket 775 800MHz
Antec Sonata III Piano Black Quiet Mid Tower Case - With 500W EarthWatts PSU as the main components along with a Samsung 500gb HDD

It has Windows XP installed with service pack 3 (at his request, he didn't want Vista - not sure why). The only problem I had was with sound not working at first and had to use http://forums.majorgeeks.com/showthread.php?t=152858 that fix to get it sorted. But it worked easy enough.

As I say the system is stable when not running games. He has used the usual stuff like MSN etc, and also has run some simple programs like PHUN.

But Crysis is causing headaches. He installed Rivatuner and GPU-Z as I didn't know which would give him temp readings. He said the GPU is around 45c when he resets his P.C after Crysis has crashed.

Other than that he has updated to the latest drivers for the GPU, and as far as we can see the sound drivers are up to date.

I did some reading and the power supply also gets good reviews and is capable of chucking out more than the 500w it's rated at so I don't think that's a problem.

But a bit stuck from here on in. I am not much of a P.C gamer myself. If games haven't ever run for me (I'm looking at you DOOM 3) then I just don't bother with them. That plus I use an ATI card and don't know anything about Nvidia stuff as I have never used them.

If anyone could give me some pointers as to what he could start checking I'd be most grateful.

He hasn't overclocked anything by the way. I did see someone on the net somewhere mention upping the voltage of the GPU by .1 but it seems you need to alter it's BIOS. As I am talking my cousin through this I don't think that's viable at the minute in case he screws something up.

Many thanks and I hope I have given enough info. It seems people with the same graphics card can get the game running fine. So I'm hoping it's some simple tweak that's needed! Cheers