Hi, this is probably really obvious but I cannot see it?!
Just bought a new PC and installed Sonar on it, however the Command Center will not let me logon to download products?
I can login online onto the Sonar site, download from there, manually, but the Command Center will not allow me to logon.
Any ideas? do I have to uninstall it off my old PC first before it will authenticate?

You can have it on multiple computers. If you haven't already...
 
1. Make sure you have the latest version of CC
2. Reinstall CC
 
If this doesn't work, try a "clean boot." There may be another program interfering/conflicting with CC.
 
How to perform a clean boot in Windows
https://support.microsoft.com/en-us/kb/929135