A PC keyboard can be used with your Mac. The Windows logo key is the Apple Command key and the Alt key is the Alt/Option key. They're juxtaposed with each other (in relation to the space bar), when comparing a PC keyboard to a Mac keyboard.
Yeah, you should be able to use that keyboard. I doubt that's your problem.
After six years, I'd reseat everything including memory and video card. I'd unplug and replug everything not already covered in your PSU exchange. Do you recall exactly what problems you were having before the machine was stored? Is this still the original hard drive? Do you have optical media to try to boot from?
To get more specific advice, give some more details about your system. Which model Power Macintosh are we discussing? From what you've written, it appears that you have an early model G5?