Windows now requires activation to prevent unlicensed use.
Windows XP and higher now use Internet activation. A mechanized phone option is available for those who do not have Internet access.
To check your activation status you can press win+r and then slmgr /xpr
Windows will then show a dialog that indicates that Windows is permanently activated.
Recall Bill Gates and his comment that Windows 3.1 was the most copied software. Windows 3.1 used 1.44MB floppies which reduced the number of disks needed compared with 720K and 1.2MB disks. Floppy disks were easily copied which is why Windows was so widely copied. The effect was to made Windows almost universally used on the PC.
If you need to change the CD-KEY you can use the system dialog. Press win+pause/break and at the bottom is the activation status and there is a link to change the CD-KEY.
If you do not have Internet, you can still install and activate Windows. Press win+x and then slui 4, then chose the country shown to get the correct phone number.