Because the license type is coded into the key itself. There's OEM, VLK, and retail, not to mention full version vs. upgrade. Just because there's a key on that laptop doesn't mean it's a full version retail key, which is the only thing you can take to another machine.
Online Windows PID Checker
That looks super useful...but I feel a little uneasy inputting something as valuable as a valid windows key into some online form, esp to a site with a .nl domain.
