I'm a bit of a windows fan-boy too, being a C# programmer one might expect that really

Thats not for want of trying, gave many linux distros a go, a few years back (before ubuntu), even tryed OSX for a few months, very user friendly OSX but a bit lack of office & visual studio, upsets me.
Still on XP everywhere, its a good OS, windows 7 is looking really good though, not upgraded yet.
To address the issue in hand, I'd defiantly say get rid of ME, worse version of windows by far, even if you leave for the dark side, its still better than ME.
The power supply is a good place to start looking for faults, hopefully its a straight swap option. power button could be a fault option, trace the wires inside the case back to the motherboard, unplug the little two prong connector and short the connection yourself, this is basically what the power button is doing, if that doesn't work its not the button at fault. As Steve says a dead battery on the mobo and it will still POST
Gone to the dark side.
Past Citroens
'99 Xantia HDi Exclusive
'99 Xantia 3.0 V6 Exclusive (Green Goblin)
'02 C5 Hdi 110 LX Estate
'98 Xantia 1.8 sx auto with LPG
'00 Xantia Hdi (110) forte
'97 Xantia 1.9D sx
'93 Xantia 1.9D lx (my first) R.I.P