PC shuts down on boot up

[Kaalm]

When I try and boot up, my PC will shut down almost immediately.

At first this was a very random occurence (first started a couple weeks ago). Thought it was may have been a heat issue but rig is reletively cool (55C for cpu, 49C ambient during peak usage). Yesterday however my PC would not even make it to the bios before shutting down.

I've tried booting from start-up disc, changing vid card and RAM but no change - PC still shuts down almost immediately. Also, PC won't start when I press key on keyboard (set up that way in bios), it will only boot from power switch on case.

I'm thinking maybe the MB is bad but no alt MB to swap in and test - would have to buy a new one. Any suggestions on what to try before I buy a new MMB (rig is only a year old).
Thanks for any help.

PC consists of the following:

MSI 865pe neo2 MB (onboard sound)
Intel pentium 4 3.0 (socket 478)
Corsair valueselect ram 512 x2
WD raptor 75 gig HD x2 in RAID
Gigabyte X800-XT 256 video
Antec neopower 480 psu
Windows XP w/service pack 2

 

[suicidalpigeon]

take everything out accept the neccessary parts to post then jump the on switch with a screwdriver?

 

[mechBgon]

Also look at the plastic frame that the heatsink locks into, and see if one of the corners broke: where to look If it broke, then your heatsink would not make good contact with the CPU, resulting in CPU overheat --> shutdown.

 

[Kaalm]

Thanks for replies,

Tried taking everything out and swapping - ram, vid card, etc w/no change. Heat sink is securely seated, no loose corners, nothing broken.

Comp has been off and is totally cool but when I try to start it up it immediately shuts down.

 

[yourdeardaniel]

switch out PSU

 

[Kaalm]

Put in new PSU, same thing occurs so can knock that off list. Thinking it msut be MB at this point. Will try reseting MB see if that works. Any other suggestions?