Computer suddenly locking up

[MintCrisp]

Ok guys i need help, 4 days ago my computer randomly started locking up, the screen just freezes, i cant cntrl, alt, delete, the cursor is just stuck and there is no acctivity from the hard drive, the only way to get out is to reset the computer, it has done it a few times whilst browsing the internet, but i cannot play any games as it locks up straight away.

Here are the things i have tried,

Run anti virus and spyware scans
Reinstalled windows
Run memtest, no errors reported
Run the seagate hard drive diagnostic program, no errors reported
Updated chipset and bios
Cleaned cpu heatsink and all fans

None of the above have worked, but i think i have tracked it down to either a faulty graphics card or a faulty psu.

If i swop the graphics card to the original nvidia 6800gt that came with the system it doesnt lock up.

So here lies my quandry, is it the card or the psu ?

A year ago i purchased a new Gainward Bliss 7800gs and it has been running perfectly up until now, but the power requirement is for a 400watt psu, my psu is 480 watts.

The 6800gt power requirement is for a 350 watt psu, it's not much difference i know but it could be that my psu is failing and doesnt have enough juice to power the 7800gs.

Heres the weird thing if i look in my bios there is no 12v rail reading anywhere, so i tried everest and again there is no 12v rail reading, also speedfan shows no 12v rail reading, could this be my problem ?

I need to know if it is indeed the psu before purchasing a new one, i have another computer that i could try the 7800gs in but the psu is only 350 watt so i dont think it would work anyway.

I purchased this rig 3 years ago and it has been running fine untill now, here are the specs

ALIENWARE AURORA-FX
ABIT AV8 SOCKET 939 MOTHERBOARD
AMD ATHLON FX55 CPU
2GB DDR PC3200 RAM
120GB SEAGATE BARRACUDA HARD DRIVE
AUDIGY 2 ZS SOUND CARD
LITE ON CD-RW DRIVE
16X DVD-ROM
480 WATT ALIENWARE PSU
GAINWARD BLISS 700GS OR NVIDIA 6800GT

So any ideas what is at fault the psu or gpu, and why is there no 12v rail reading anywhere ?

Many thanks.

[Jimmy 2004]

Don't have a clue why there is no 12V, but I would also suspect the PSU - first of all ensure that all the power connections are nice and firm.

I recommend that you try running Prime95 for a couple of hours with the 6800GT installed. If you get errors your CPU could be on the way out. Have you been overclocking at all? If you have a friend you can borrow parts off that would be useful so you can swap them into your system, but at the moment it looks most like PSU, possibly graphics but it could still be the CPU.

[ktr]

My comp been doing the same while gaming. I thought is was memory, but memtest said it was clean. So I reinstalled XP and my unit is working flawless. I have a hunch on one of M$ updates...hmm...

but you have already reinstalled everything, it seems to be the psu then...

[JC316]

It really seems like you did your homework on this one and I commend you for that.

I also say it's the power supply, you have narrowed everything down and since it's an off brand, I would say the PS is cheap. I would pick up another power supply. The Antec Smart Power 2.0 500W is a damn nice unit and it's only about $69 bucks from newegg these days.

[MintCrisp]

I have finaly found out what was causing the lockups, it was indeed my psu and not the graphics card.

The lockups started to get more frequent, so i started removing things from my system one at a time until i was left with just a bare bones setup, and still the lockups persisted, i then removed the psu and found out that 2 of the pins on the molex connector to the motherboard were badly burnt, infact the plastic coating crumbled when i removed it.

So i have replaced it with a new 700watt psu, probably overkill but better to be safe than sorry.

One thing puzzles me though, there is no reading from the 12v rail anywhere, not in the bios, not in speedfan and not in everest ?

Even with the original psu there were never any readings for the 12v rail, has anyone come across this before ? is it a quirk of the motherboard, i have asked a few people and they dont know.

[Jimmy 2004]

Must be the motherboard - it's either a minor fault or a missing feature, a BIOS upgrade could help it. Glad you've got your system working again, I wouldn't worry about there being no 12V reading if you've got a good PSU. What make/model is it?