If you can't get MC15 to start because you've installed IE9, then you should be able to do this (Vista or Win7 Instructions):
0. Make sure MC isn't running in any way, including the Library Server.
1. Go to Start and type: regedit
2. Hit Enter.
3. Approve the UAC prompt if necessary and wait for the Registry Editor UI to load.
4. Navigate to the following registry key: HKEY_CURRENT_USER\Software\JRiver\Media Center 15\Properties
5. Find the Startup - Page key in the list on the right-hand side.
6. Double click the Startup - Page key and change the Value data from 0 to 1.
7. Click OK and then close the Registry Editor.
This will set MC's startup location to Playing Now instead of Start.
This will allow MC15 to start up properly even with the IE9 RC installed. Then, in MC, you should go to Tools -> Options -> General -> Features and uncheck the "Show Start in Tree" option. This will disable the start page in the tree. You may also need to disable other web features (like Connected Media) if those features don't work. The Wikipedia lookup feature probably won't work either, but I don't think you can disable that.
Lastly, to fix the Links bar so those items work right, you'll want to make them open up in your system default browser instead of the internal browser. Select one of the items in your Links Bar (like IMDB or AMG, for example) and choose Manage Links. Then change the Action for each of the Links entries in the list to Open in external browser.