PLEASE HELP !! PC crashes few minutes after booting

I have a PC with this following configuration

Motherboard - ASUS A7N8X-E Deluxe
NVIDIA nForce2 Chipset Rev.68.02

CPU - AMD AthlonXP 2500 Barton processor

RAM - 1 GB (512 333 + 512 PC2700)

Problem:
I boot the machine... it runs for a 10-15 minutes and when I run an application, burn a CD or play a game etc.., it just switches off automatically. After than I will need to remove the power plug from the socket, wait for few mins and then restart.... after a while the same problem reappears...

I had my PC built about a year ago... It was working fine for few months initially. It had ABIT NF7 motherboad.. One day all of a sudden it switched off automatically while I was working on it. I took it ot the hardware engineer and he said the mobo was faulty and he replaced ABIT with ASUS A7N8X-E one. That worked for a couple of weeks but then it crashed again.. The seller didn't take the mobo back.. I talked to couple of my friends... they said the problem is with the PSU. I had 300W power unit and yesterday I changed it to a new one with 450Watts. I checked the fan, it's running properly.. I can feel the heat on the back of the PSU but I guess it's not too hot... so I think the problem is not related to temperature.... but not sure.... I am not a hardware pro.. and my options are very limited. I am very upset and now I have this damn PC lying with no use. I spent a lot of money on it and can't use it.... so frustrating..

I am a new member of this website and I am hoping that I would get some help from mobo pro's.

Please help!!

What should I do. I sent an email to asus CS two weeks ago but no response.

Thanks
Naga