First, let's make sure MC is not running:
Check the processes tab in Task Manager (not the Applications tab), see if there's a Media Center 12.exe process.
If there is, but MC is not in the Applications tab, you'll have to kill it. But - let me know if it is there, that's a problem.
Next, let's kill the lock (make very very sure that MC is not running, either as an application or a process).
Take a look at your library directory (where the library is stored, not where music, images, and video are stored, this is probably
C:\Documents and Settings\skylarplane\Application Data\J River\Media Center 12\Library) and see if the file "lock.jmd" is there.
Remove it if it is. Let me know if you cannot remove it, that's a problem.
Now start MC (it should start without the message, if it does, let me know, that's a problem). Use it for a bit if you'd like.
Then, turn on debug logging, clear the log (under Help->Logging...) and then exit Media Center.
Now, check for the process and the lock file again. Start MC (ignore the message) and look at the previous log, post it and the details here.
j