1.) Hit Start
2.) Hit Run
3.) Type Regedit
4.) Find HKEY_LOCAL_MACHINE and click the arrow to the left of it.
5.) Find SOFTWARE and click the arrow to the left of it
6.) Find EAGAMES OR ELECTRONIC ARTS click the arrow to the left of one till you find command and conquer red alert 2 yuri's revenge.
7.) Under one or both of EAGAMES or ELECTRONIC ARTS you'll find that now click the arrow left to it and find ERGC
8.) Now to the right it should have your CD-KEY. You can now change it...Just click it twice it with popup and allow you to edit it.
9.) Once done editing hit OK then close out regedit. You should be now good to go