new mobo wont boot and can't reinstal vista

HI,

I have 6 PC's, one old desktop and one old laptop on XP. One new laptop
upgraded to Vista-U. Two desktop's upgraded to Vista-U and one micro-desktop
running XP-MCE. (yes, all with their own licenses)

What was my xp desktop machine was upgraded to vista business and then to
vista ultimate (don't ask why !). Recently my motherboard melted so I
replaced it and the cpu however it refuses to boot and 'startup repair' does
nothing.

Here's my real problem... I can't reinstal anything since I don't have the
xp discs etc anymore and any attempt at the double-instal trick with the
ultimate 25char key gives the "can't instal this from that" message...

Any suggestions ?

Thanks,

Jason.