Another POSTless computer.

I'm sorry to put another one of these in the forum, but I'm not seeing anything that's helping in other posts.

For the past few days, my computer has been shutting off at random times until finally it just stopped booting up altogether, no POST or anything. The fans come on and within an instant shut back off. I believe that it isn't POSTing because I had to take it into a local computer store once before and that was the problem with it not booting up. I would really appreciate help with this.

AMD Athlon 2500+
MSI K7N2 mobo
1GB Mushkin RAM
ATI Radeon 9700 Pro
Sound Blaster Audigy 2 Gamer
Generic 10/100 Ethernet Card
Samsung DVD-ROM drive
Samsung CD-RW drive
Belkin 3 1/2" floppy drive
Samsung 160GB Ultra ATA hard drive

I had a computer that would reboot sometimes when the refrigerator started; only sometimes when it started. I ran a new circuit just for the computer and it still did it. I final put an oscilloscope on the +5 volts from the power supply and saw a significant ripple. I changed the power supply and it fixed the problem. My conclusion: sometimes the fridge would start up when the ripple was at a low point, and the current draw from the fridge starting pulled the power supply down even further causing a reboot...just a guess though.