And don't forget that you can't transfer a Windows OEM license anyways. It's linked to the PC you purchased forever, it cannot be transferred to another computer. Even if your original PC dies the license is lost unless you repair the PC.
You need a "retail" version of Windows to be able to transfer the license from one computer to another. But even with a retail version, it cannot be installed on 2 computers at the same time it's illegal. If you want to transfer a retail license you need to uninstall it on the other computer first, otherwise if you use the same serial number on 2 different computers Microsoft will detect it and block your serial number so you won't be able to do updates anymore.
Usually you call Microsoft for Activation only when you run into problems. It's so much easier to do the activation in Windows through Internet, only have to click one button and it takes only a few seconds to activate.