Upgrading Motherboard, processor, graphics card new os?

Yes in a sense. If you replace the motherboard of a working windows machine, you need to "repair"(not a fresh install) the windows installation. It wont work until. This is because the new motherboard is an entirely different thing for windows in your HDD and it wont be recognising anything in that. If you have windows recovery DVD(which is normally supplied with the PC) then do a repair of windows by booting from it. Dont worry about your data because the repairing of windows only replaces the files in C:\windows(or wherever ur windows installation lies). If you dont have recovery CD you can borrow a windows CD(make sure that it is the same version, say if you have windows 7 64 bit, get the same home 64 itself) from your friends who have it

Not necessarily. I replaced my mobo and Windows booted just fine without a repair. All I had to do was let it activate the license again. This will not work with an OEM copy of Windows though, since the licence is tied to the hardware.

@herr_koos; hmm... i havnt checked it with a retail-copy-installed-machine.. I have actually tried in hundreds of machines(OEM copy installed) with the same scenario(ie motherboard up-gradation) but none of them have worked like in your case.. I had to give "repair windows" or in worst cases, fresh install always...

I guess I was lucky then. :-) But yes, I suspect mine worked because I used the full retail licence.