I am using Windows 7 Professional. Now if I want to change my processor or motherboard, is it possible to use the same licence key on my PC?
If it is an OEM version of Windows 7, then CPU and motherboard upgrades will violate the EULA. So, you may not be able to activate it. OEM copies of Windows are tied to the motherboard/CPU they are installed on first.
If you are using a retail Windows version then you can move it to a different computer as many times as you like. Of course, it can only be installed on one computer at a time.
If you do want further clarification you could always call Microsoft and ask them directly.