There's one advantage for not changing mobo: you don't have to reinstall everything again. Of course, that can be a good thing too if you want to start fresh. But if you just want what you already have working a bit faster, dropping in a new CPU is a lot easier than rebuilding the whole thing.
Having said that, I think Win7 is more tolerant of major hardware changes, but generally I'd consider any particular install of windows as associated with a mobo.
_________________Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.