from media player help:
You cannot copy and share licenses among different computers. If you copy a licensed file to another computer or give a copy of the file to another person, you or the other person will need to acquire a new license for that computer. This process starts automatically when you play the file on the new computer.
To restore your licenses
On the Tools menu, click License Management.
If required, insert the floppy disk on which your licenses are stored.
Select the location of the backup copies of your licenses, and then click OK.
In the License Management dialog box, click Restore Now.
Follow the instructions on your screen.
An Internet connection is required to restore your licenses. You can only restore your licenses a limited number of times.