since you're going to scavenge a prebuilt system, its windows' license is tied to its hardware and the changes/upgrades you're making will require re-activation. since you can't activate a dell/hp/gateway/etc on other hardware, you'll thus need a new copy of windows. also, your current install probably won't even boot with a new motherboard since the low-level drivers will be for different hardware.
I am building a new computer, but wanted to use some of my current components like my DVD burner, DVD/CD-RW and the old Hard Drive (80 gigs) and upgrading with a new Motherboard, Possessors, Video card, Sound Card and PSU. My question is if I change all of this will i have to reinstall my windows, the old HDD as a recovery partition and the computer didn't come with any disk to reinstall you just have to hit F11 at the first screen that comes up and then it will completely wipe it clean and reinstall everything that it came with from the factory.