Hi ..... I've got a mac G4 dual boot ( one of the last made before the G5), so it's way outside its warranty/support package. I've been trying today to register, finally, with garageband but it's not playing the game at all. It tells me - having suggested that an email address is the most usual form of Apple ID - that my address is invalid/not recognised. Any ideas of what I can do to get round this pesky little problem?

The Apple ID login can be problematic at times (I assume you mean registering via the web site, right?). I've found in the past it helps to empty your cache3 and restart your browser before putting in your Apple ID. And yes, an email address is the most common Apple ID AFAIK, but you can have other ones too.