Replace motherboard with exact same model BUT change processor?

you should not have any problems with Win7 even if its an OEM version. MS can reset if you have to call don't worry, just call them and explain what happened they are not going to crawl through the phone line and tear your head off or anything.

You'll probably have to re-activate Windows. (Even if you kept the same CPU, the MAC address of the onboard ethernet controller will change, and that's one of the factors likely to trigger activation.)

I'm not expert on OEM licenses, but it sounds like you're well within the requirements. The worst case is that you'll have to activate by phone, and you might have to speak with a human. It might cost you 10 minutes of your life.

This leaves the question of whether Win7 will start on the new hardware, which you cannot count upon when a major piece is changed. There are two approaches:

1) SysPrep Win7 on the old hardware before changing it, to remove all drivers, SID, HID, activation so that it will start on any new hardware the HD or its image is moved to. SysPrep to move HD to another computer

2) Attempt to start the HD on the new hardware. Since it normally fails at boot animation due to a convergence of hardware driver and SID/HID changes, it may start. If not you can boot Paragon Adaptive Restore CD to Adapt OS to New Hardware. PAR is not freeware.

Shouldn't be a problem, I went from an ASUS M2N68-AM SE2 to M4N68T-M V2 without having to re-install or sysprep or anything like that. They both have the same chip-set, only driver I had to un-install was for sound. One was Realtek and the other VIA. I was prompted to activate, and it activated fine on line. Likely the result of the different sound chips and going from DDR2 to DDR3. Windows 7 x64 off the shelf OEM version. I'll bet you won't even have to activate seeing as its the exact same motherboard.

I've just changed the motherboard with a new one. And of course a problem. System attempts to boot from D: \ windows instead of C: \ Windows. Now it's C: drive "System Reserved" and the D: drive is actually a former C:. The solution? I tried to replace the Active partition, but then it does not boot.

Hm, thats weird that the drive letters swapped. Did you match BIOS setting on the new one to what was on the old one? Hard drive connected to the same SATA port?

Do you have a configuration where System Reserved partition was booting Win7 on C?

If so boot free Partition Wizard bootable CD, rightclick on System Reserved to Modify>Set to Active, OK.

PW CD will normally not show drive letters, but if it does, remove the drive letter on SysReserved partition as it is supposed to be.

Next click on the Win7 HD to highlight it, from Disk tab select Rebuild MBR, OK, Apply all steps.

If Win7 doesn't boot now, boot into the Win7 DVD REpair Console or Repair CD to run Startup Repair - Run 3 Separate Times

If Win7 will not boot from SysReserved, boot back into PW CD to set Win7 partition Active and Rebuild MBR, then run Startup Repair x 3 if necessary.

Last resort you can attempt to reconstruct the MBR by deleting SysReserved, recreating it Primary NTFS, no letter, marked Active, run Startup Repair x3 to write a new SysReserved partition including the Repair console it places on F8 Advanced Boot Options

gregrocker,
that's it. I hid a partition and normally the system is started.
Tnx