GarageBand missing folder

[Edward Arrowsmith]

When I run GarageBand lately it fails to launch and I get the following 
message:

The Instrument Library is invalid. GarageBand expects a valid 
Instrument Library folder in /Library/Application Support/GarageBand.
Please choose a valid Instrument Library folder or quit and re-install 
GarageBand.

I can't find a valid instrument library and I can't install GB from the 
install discs. Any suggestions please?