I just installed Kubuntu 12.04 alongside Win7. I have done exactly this before, but my hard drive had to be replaced (unrelated reasons) and I had to start again.
Install went through smoothly, I rebooted the system, when I press enter on the Win7 entry, the splash screen comes up but freezes on 'Starting Windows'.
Kubuntu boots fine.
Running 'bootrec /fixmbr' from the Win7 DVD fixes the MBR and Win7 boots fine after that, suggesting no corruption on Win7.
Something GRUB is doing(Im guessing near the boot) is preventing Windows from booting properly.
Can someone please help me figure out what could be wrong?