If you are just changing a few bits of hardware, it shouldn't require re activation.
If you install over your current installation, you may require reactivation.
If you do a fresh installation, you will need to reactivate.
On top of this, if it is only a few times in a long period of time, it is unlikely you will have problems. If however it fails to activate, simply ring the toll free number it gives you as an option and explain the situation to the operator. They will waive you through and either give you a new serial key or just let it through this once and reset your acitvation counter.
The fact is, activation is horrible but Microsoft do not do it to catch out legitimate users who need to reinstall - they do it for the people who leak a serial key and thousands of people use it. As long as you have a valid licence key and explain, they will let you have as many installations as you need* - no problem.
*as long as you abide to the EULA and not more than one at any one time!