I've upgraded my PC from Core 2 Duo E6600 + ASUS P5B-VM to Core i5-2400 + Intel Motherboard. I'm using the same HDDs I used in old PC. After the assembly of new PC with old HDDs (One HDD had a partition where Windows 7 Ultimate was installed), I've noticed that Windows 7 failed to boot. I tried to fix startup problem with the recovery option of Windows 7 installation disc but it failed to repair. It looks like because I change either the processor or the motherboard for which Windows 7 failed to boot. Then I had to reinstall Windows.
Now I'd like to know why the new system couldn't recognize the OS from HDD? Why even the repair failed? I'm a Computer Science graduate and a programmer. So you can answer technically.
Note: I've also upgraded my RAM from 3 GB DDR2 to 4 GB DDR3 but I believe this doesn't matter.