Werid Problem not sure whats causing it?

Right me and my dad know a lot about pc's but this one is really getting on my nerves now i upgraded my pc with a new motherboard cpu ram graphic card. they all should work as everything fits fine (not pushed) the problem is that it wont boot windows XP home Sp2 or even Win ME it gets as far as u didnt start it up right and when the splash screen comes up BOD plus i cant reinstall windows as the pc freezes when checking my hardware to see if windows will work.
Spec's
CPU = AMD 2.4GHz Dual core AM2
Graphic Card = Ati 512Mb DDR2 Radeon X1300
Ram = 2x 1Gb DDR2
Motherboard = Gigibyte S series (GA-M55plus-S3G/2.1)

Now ive had the motherboard replaced as they (the company where i brought the stuff) thought it was that but still dont work and ive tried a different CPU Ram and Harddrive in there and still no luck even with the onboard Graphic's incase it was the PCI-E graphics card messing.
I hope someone out there will know the fault if u need more info i will try to provide it.