I was just about to mention this. I used to work in a mom-n-pop shop, the only one in the area, for a long time.
I have seen some of the most ridiculous problems that were PSU related. Serial mouse not working, VGA card outputting in B&W, slow and or intermittent performance, HD's that constantly reset (and sound like click of death in the process), new memory being blown, known good memory acting like bad memory, CD-R's that can't burn (or finish burning successfully), software modems that couldn't go off hook, AGP cards crashing, PCI cards crashing, VLB SCSI cards not working at all.
The list really just goes on and on and on. Software to diagnose faulty PC hardware? Sorry, no thanks. I had tried all manner of diagnostic and test software over the years. Some worked some of the time. (mem tests and HD scanners), the rest were borderline use-less pieces of crap. Not only that, but because of faulty PSU's (usually overloaded, or just old, or overheating, etc etc etc) I have seen those same programs misdiagnose just about everything.
Aside from simple sensor reading and verification (of code, built in HW diagnostics, etc) I do no trust 'software based' hardware diagnosis, especially on a PC.