If you wish to save yourself some aggravation while testing this release, I would suggest the following after you've ensured that q3key contains your valid CD-Key;
[...]
open up "My Computer" and head over to the directory where you have Quake 3 installed. Open the "baseq3" directory, right click on the "q3key" file. Select "Properties" and on the bottom of the panel, you should see a "Read-only" check box. Check that, and hit "Ok" to apply the changes.
[...]

Of course, considering there will be another release in a week or so unless you're feeling saucy and just wanna check out the latest build, I'd recommend just waiting for the final v125 release.