I bought all of the components for my PC a couple of years back, including a nice shiny new copy of windows XP to install. I figured that's my copy to use as I see fit. I paid for it. It's only used on one PC.
Anyway, I have good news today. Just got back from Maplins, I bought a new uprated power supply. I lobbed it into the case. The Power supply that is. Plugged it all in and turned it on. It works! So it was a power problem.
Thanks for all of the info so far though. All stored away for future reference.

