computer swithces off on its own

Hey,
Can someone please help me with this problem.
Every time I use my computer for some intensive work like rendering a video my computer switches off automatically. However this doesn't happen normally. It only happens when i am converting a video from one format to other or rendering something. I have already installed 2 fans. Also got my processor fan cleaned up. I have a 1.5GB ram,Pentium 4 D865GBf board.
Please help me sort this out.
Thanks.

Start by running Memtest86 to test your memory. What is the power supply's wattage rating? This may be a power supply issue too...

Video encoding is one of the most cpu intensive tasks out there. If you run an over clocked system, it is a good indication of a bad/unstable OC.

these are the temp readings:

motherbard: 38 degrees
cpu : 40 deg
aux: 52 deg
hard disk :48 deg

cooling fans
cpu: 4131 rpm

voltage values
cpu : 1.46 v

Memory test:
Physical memory: 1534 mb
swap space :2156 mb
virtual memory : 3690 mb

Ram: 2nos
1. 512mb ,166mhz
2. 1024mb,200mhz

1.46 v-core? Have you tried running your system at normal clock-speed to see if the problem is still there?

I havent overclocked my system at all.