Shutdown on startup

[gijoe9482]

Hey everyone, just a little->big problem.
I've just built a computer for myself but for some reason it keeps shutting down with no warning on a cold startup.
It boots up okay, but when i open a low processing program like microsoft word or internet explorer it shuts down with no warning. The CPU is not over heating because the motherboard sounds 2 beeps and then shuts down. In this case it just shuts down.
To over come this annoying problem, what i do is bootup and then wait for 5 minutes before i do anything so the computer can warmup. This is so annoying. The computer runs fine after warming up. Can anyone help?

My computer specs are:
CPU: AMD Athlon XP 3200+ 400Mhz FSB
M/B: Gigabyte 7VT600-P-RZ
RAM: 2 x 512MB DDR 400Mhz (unbranded but fully tested and working)
Graphics: ATI Sapphire Radeon 9200SE Atlantis
Sound: Q-tec 555S 5.1
TV-card: Pinnacle PCTV rave
PSU: Q-tec 550W Dual Fan Gold
Primary HD: IDE Maxtor 80gb
Storage HD: IDE Weston Digital 250gb
CD-rw / DVD combo: GCC-4480B
DVD-rom: samsung SD-608

All the drivers, firmware, and bios are up to date.
Have i built this computer wrongly or something. Any help would be fully appreciated.
Thank you in advance.