Power supply problem?

[glatt]

My home PC is having intermittent problems. How can I tell if it's something like a power supply that I can fix?

I don't remember the model. It's a 5 or 6 year old Dell desktop that was very good in its day. Sometimes, when I go to turn it on, I push the power button, and nothing happens, except the button flashes amber. It's supposed to turn solid green. There are 4 diagnostic lights on the back, and none of them light up, which according to the diagnostic stuff on the web, means the PC isn't plugged in, or it's got some pre-BIOS error. (I think that's what it said, but it was 2 weeks ago that I looked it up.)

When I unplug the PC for 5 minutes or so, and then plug it back in, it has started up again when I push the power button. My wife just called and said that she tried that trick and it didn't work.

I'm guessing, with no knowledge and little evidence, that it's a problem with the power supply. My experience working inside computers is limited to adding RAM to this very same PC about 3 years ago. I have some confidence, but I have very little experience with electronic circuitry. I own an auto ranging digital multimeter. Is there any way I can see if it is the power supply? And if it is, are they easy to replace?

I should add that a couple weeks ago, it got infected with some virus or other malware, and I spent an evening of running scans over and over before they fixed the problem. I thought it fixed the problem, but maybe not?