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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.

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up vote 4 down vote accepted

Because all the hardware infomation your old install used to load has changed, it does not have all the information and driver files it needs to boot properly. This can be especially true if the system does not have a disk driver for the new motherboard. It won't even get past that.

Since you aready reinstalled, this won't help, but in the future boot your Windows 7 disk, and choose the Repair option. You will see a screen like the one in the screenshot. At this point, it sounds like you will need to load your disk driver. Hopefully after you hit next, it will fix any problems you may have.

enter image description here

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Hit cancel when it asks you about system restore. – KCotreau Jul 2 '11 at 15:42
This will not fix it most of the time, when hardware is too different. They did not give us a way to truly repair an installation like XP had. Reinstall time. – Moab Jul 2 '11 at 15:50
I have not done it enough to know how often it will, or will not, work. While I would try to just load the HDD driver initially, I would think that if you loaded more of the more critical drivers, like for the motherboard, your chance of success would probably go up. – KCotreau Jul 2 '11 at 16:33
@KCotreau: I didn't try load drivers. i didn't know what it does before reading this post. however, i tried to repair windows but it didn't fix. i went through several iteration of repair attempt but windows 7 couldn't repair it. – Donotalo Jul 2 '11 at 18:09

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