Hang on a minute. I didn't spot the link to 'rebuild a corrupted search index' until just now. I rebuilt the index as suggested, which took 5 mins, and behold! Media player now has everything recognised and in the library. I don't know why the search index would need rebuilding, it's an almost new machine which hasn't crashed or had any other problems as yet, but it seems to have done the trick.
Thank you guys. I would never have thought of Windows indexing. If I remember back with XP I disabled the indexing function to increase the processor speed and everything still worked fine. This new machine has Windows 7 running in its default state (bags of processor speed to spare) so I assumed all would be ok.
What I need to do now is to see if the next Cd I rip is automatically recognised or will need another index rebuild.
Computers! You gotta love 'em!