Adding upgrades to my computer

Is this a retail copy of Windows 7 or an OEM copy that came preinstalled on the computer?

If it was a prebuilt computer from, say Bestbuy, then technically an OEM key will not activate on another machine (another machine is considered a different motherboard). Microsoft may take pity on you or the phone activation might if you try to activate it. If you can get a hold of someone and explain your situation then they may also activate the key; however, you take the risk of the Windows not activating if you attempt to put it on another motherboard.

In your case, if you only want to upgrade parts that are not the motherboard, the license should still be valid. No one knows exactly what triggers the reactivation flag. If you change these parts and the Windows flags you to activate it, then I would call Microsoft and explain your situation and tell them that the motherboard has not been replaced, only the parts that you mentioned.

This is straight from Microsoft's website.

Q: Can a PC with an OEM Windows operating system have its motherboard upgraded and keep the same license? What if it was replaced because it was defective?

A: Generally, an end user can upgrade or replace all of the hardware components on a computer—except the motherboard—and still retain the license for the original Microsoft OEM operating system software. If the motherboard is upgraded or replaced for reasons other than a defect, then a new computer has been created. Microsoft OEM operating system software cannot be transferred to the new computer, and the license of new operating system software is required. If the motherboard is replaced because it is defective, you do not need to acquire a new operating system license for the PC as long as the replacement motherboard is the same make/model or the same manufacturer's replacement/equivalent, as defined by the manufacturer's warranty.

The reason for this licensing rule primarily relates to the Microsoft Software License Terms and the support of the software covered by those terms. The Microsoft Software License Terms are a set of usage rights granted to the end user by the PC manufacturer, and relate only to rights for that software as installed on that particular PC. The system builder is required to support the software on the original PC. Understanding that end users, over time, upgrade their PCs with different components, Microsoft needed to have one base component "left standing" that would still define the original PC. Since the motherboard contains the CPU and is the "heart and soul" of the PC, when the motherboard is replaced (for reasons other than defect) a new PC is essentially created. The original system builder did not manufacture this new PC, and therefore cannot be expected to support it.

Again, in your case, as long as you leave the motherboard alone, you should be good as you have not created a new machine. If you wish to upgrade your CPU, you will have to stick with the same socket that is on your existing motherboard.

http://www.microsoft.com/OEM/en/licensing/sblicensing/Pages/licensing_faq.aspx#fbid=R5eVWKlA6wq

If you purchased a full retail version of Windows, you can do what you want with it and transfer it as many times as you wish.