I was trying to install Sonar Producer 8.5 on a new computer with Windows 8.1. The product installed (seemingly) ok but when I go to start Sonar I get the little Complete Registration window where you're supposed to enter the Registration code but in the box for the registration code it says "Not running as Administrator". Is there a known workaround for this? Which leads me to the next question: is Sonar 8.5 compatible with Windows 8.1 or am I wasting my time trying?

You should run it as an administrator.   

In the user account I am set up as an Admin. I'm the only user on this machine. Or am I missing something?

Yes you are. From Vista onward to prevent malicious software from infecting your machine individual programs need to have more permissions to write to disks and such. You can do this in an executable's properties or simply right click and select run as administrator.
 
   

Got it thanks John!