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A tech support person fixing my laptop told me that the laptop's motherboard is damaged. At the same time, he called me and requested the Windows password so that he can access the computer.

Is there any contradiction in this story? If he changes the motherboard to a new one and connects it to my old hard drive, is it possible to access the OS on the hard drive?

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if he change the motherboard with new one and connect it with my old hard drive, Is it possible to access the old windows ?

Short answer: Yes

I have swapped out a motherboard and been able to access the Windows install that was on the hard disk in the system prior to the motherboards.

In hundreds of cases, I have advised technicians I was supervising to swap out motherboards as well. In all cases where the existing hard drive was not damaged prior to my technician arriving on the customer premises, the Windows install was accessible once the failed motherboard was replaced.

The only possible hang-ups I can see would be if the motherboard was not very similar to the old one and your system's Windows install was under an OEM license. In cases like this, you or your technician may need to contact Microsoft for assistance with activation. In certain circumstances, you may need to purchase a new license.

Unless you have been advised that there is to be a licensing issue, it is reasonable to assume that there will be no such issue.

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Even if the hardware was different. There are ways to boot to entirely new hardware using an existing Windows installation. You basically have to remove all the drivers from being loaded. Of course this doesn't explain why the password is required, the fact Windows was able to get to that point, proves the installation is working. – Ramhound Feb 21 '13 at 13:29

Yes, it's absolutely possible, and incredibly likely. The only thing that might prevent it would be some sort of drive encryption software. Barring that, hard drives are pretty much interchangeable.

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but the booting information and parameters for the windows are stored in the last motherboard' bios, how could he access the windows in the new motherboard without booting informaiton stored on it ? – oofy Feb 21 '13 at 1:01
That info isn't stored in BIOS. It's stored on disk. – cpast Feb 21 '13 at 1:06
@cpast when the cpu start booting and loading the operation system , he read the contents of a specific memory address that is preprogrammed into the CPU. In the case of x86 based processors, this address is FFFF:0000h. The code that the processor reads is actually a jump command (JMP) telling the processor where to go in memory to read the BIOS ROM. after loading the bios info, he goes to read from disk – oofy Feb 21 '13 at 1:10
@oofy And? Your point? – cpast Feb 21 '13 at 1:12
Bootloaders have been standard for many years (think decades) and all BIOS that I am aware of is able to find the primary bootloader on the hard drive... @oofy, you are speaking about a secondary bootloader, which is stored on the drive. The primary bootloader will locate and run it. No worries. This happens all the time. – kmort Feb 21 '13 at 1:35

A motherboard, while essential to run the computer, doesn't store much of anything. The only information contained on the motherboard, in fact, is technical elements of the hardware configuration (and the clock). Everything else goes on the hard drive; in fact, it is possible to take a hard drive out of one computer, put it in a different one, boot the second one, and use it with no issues at all. I have actually done this (when a server died, I removed the disks and put them in a different server); while there are minor things that may not work, almost everything works fine.

Specifically, the reason this is possible is that a lot of work goes into standardizing things so that it is possible. There is a standard way to store bootloaders; every motherboard knows how this works, and so can boot from any hard drive.

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You can do this with all operating systems. In most cases operating systems are installed and configured to load certain drivers, this often can cause problems, even with Unix and Linux although in those cases there are easier to resolve. – Ramhound Feb 21 '13 at 13:33

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