At one point I had someone rename rename the Admin account.
Maybe, this has messed up the permissions on your system.
Check, if your Instrument Library exists and check the permissions. The instruments library should be here:
Your Macintosh HD > Library > Application Support > GarageBand > Instrument Library
- If the library is there, but GarageBand cannot open it, use Disk Utility to repair the permissions on your System volume. Disk Utility is in the Applications > Utilities folder. select your disk in the left panel and click the "First Aid" tab to access the "Repair" tools.
- If the Instrument Library is missing, you need to restore it.
On MacOS X 10.4.11 you have probably installed GarageBand using an iLife installation DVD.
Either restore the Instrument Library folder by copying it from your backup or use your iLife installation DVD/CD to reinstall GarageBand. That should restore the missing Library as well.