Thanks for all the suggestions, comments and info. Thanks particularly for TreeSize - I used to have this years ago and forgot how simple it was.

I've done a bit of digging using TreeSize, PerfectDisk and CA Antivirus.

TreeSize reports 83,652 files and PerfectDisk 92,335 files, which seems sensible. The difference is mainly system files, some of which are not reported by TreeSize. Breakdown of the PD figs as follows:

C: (Windows) 72,976
E: (Photo)    3,919
F: (Data)    5,222
M: (Music)    2,017
Q: (Backup)   3,854
R: (Backup)   4,347

So where are my missing 500,000 files? Well, a good chunk are in 24 .CAB installation files for Open Office. CA Antivirus counts all the files hidden in these, and also all the separate files in zip archives.

I feel lots happier now!

Phil