My problem is after the progress bar for downloading the apple loops is completed my application shuts down, and when I reopen the application the message appears again and the progress bar starts over.
Are the loops and instruemts actually downloaded? Check the System Library.
You should see the additional contents in
/Library/Application Support/GarageBand/Instrument Library/
If everything is there and properly installed, check
/Library/Audio/Apple Loops Index/
There might be a file "Update Index.something.txt". Move that file to the Trash (but don't empty the Trash) and try again to run GarageBand. The problem might be, that GarageBAnd cannot write the Update result to the library.
If that does not help, run the installer manually as described by mteep: