A week ago I build a new comp. Worked fine, decided to go legal and purchased several licenses of Vista. All installs have been flaky, mainly with power. So I got tired of Vista and went back to XP.
The machine in question was fine, XP installed fine. Everything looked great. And then "blip", power turned off. And the power switch doesnt do anything. If I wait long enough, sometimes a few minutes, I hear this little "shhwip" noise, and it lets me turn it on again.
But it only lasts a few second. I have checked the cpu cooler, and all the connections. And I know that the system was working fine earlier today on XP, and previously for just over a week on Vista.

CPU: Pentium 4
Case: Antec Sonata II w 450W (stock)
Ram: 2 Patriot 256MB Strips
Video: 2 Radeon X1300's dual ouputs. (1 PCIE card, 1 PCI card)

All parts were brand new, and as mentioned, the machine was working for over a week, without hiccups.

Any ideas?

I think you've set your system in hibernation mode. Run your computer. Try to press your power button for about 10 sec. in order to discharge everything in your system. Or your bios settings has been changed. Try to reset your cmos settings in default values.

yea, the issue now is that it does not power on long enough for me to do that. The perplexing thing is that hardware-wise, nothing has changed. And the system did work for a week. Software -wise, I did a fresh install of XP, to get rid of Vista. XP was running successfully after the install for about 3-4 hours.

A computer that shuts off at random times is often a heat related issue.

Check carefully your cpu fan again. Add another case fan to dissipate some heat inside the unit.

If it doesn't help, clean your memory stick with a pencil eraser and reseat it several times in its slot or try to switch it in different slots. Or maybe your computer is suffering from a ram compatibility issue.