Bad PSU, or is it just a bad powerswitch, or worst?

Sounds like the PSU and the easiest to check is to install a different PSU. The problem is that Dell often uses proprietary PSUs, which means a standard PSU won't work or could damage the mobo. There are PSU testers that aren't very expensive but can't recommend one. I'd google PSU testers and see what comes up. cyberguys.com carries, they're like the online Radio Shack for computer stuff. Hopefully, someone else could recommend a good inexpensive one. As far as installing a new PSU, really easy, 4 screws to remove the PSU and plugging in the board and parts. Just check out it's wired before you remove the old PSU

Thanks for the heads-up on the "proprietary" aspect. I appreciate your imput.

Not much need to worry about the "proprietary aspect" with a DX051.
It has an open back design that allows the use of most standard ATX units, and Dell did away with the non standard ATX pinout with the introduction of the P4.
Dell's 375w psu is taller than standard so there will be a gap left when installing an aftermarket unit, but the holes still line up.
Here's a good upgrade;
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

I agree. Do not worry about it, a standard ATX should fit. Why not call India and have Dell do warranty on your computer over a 1-900 line   Just kidding.

Anyways I'd get a new PSU. Even if that's not the problem, you want a good PSU. The most important part of any computer is a quality power supply, not the CPU speed like most people believe. The standard DELL ones are crap. The Antec mentioned above is fine, or even upgrading to an Antec NEO with modular cables is even nicer.

The only possible gripe you're almost certain to have with that Antec Earthwatts PSU is it has a loonnngg cables. If you don't care or can stuff them somewhere in that Dell case, it is a nice steady and very quiet PSU.

I put in an Antec "Earthwatts" 500w (loong cables included) and everything seems to be back 100%.
Thanks everyone for your input.