Progress Report:
I'm trying to set up X64 so that I can use it instead of X86 and therefore take advantage of the currently unused memory.
Anyway, I appear to have got the two O/Ss working as I want although I still need to install a load of software under X64.
I managed to fix the access problems by adding "Everyone" security with "Everything" to the folders, etc. on D: - not very happy about this! What I couldn't understand was that this was unnecessary on the K: partition (separate disk drive from D:).
And then I noticed that Windows Explorer had "changed" the name of the D: partition to "Local Disk" and wouldn't let me change it back. Disk Management still showed it as "Data Disk". This was evident in both O/Ss.
After a bit of thought I realised that the only difference between K: and D: (that was evident to me) was that I'd added folders on D: to the "Libraries" in Windows Explorer. So, I removed these folders from the Libraries and at some point in this procedure (presumably after removing the last one, but I was also rebooting between the O/Ss during this and can't say when categorically) I realised that Windows Explorer was now showing the correct partition name for D:.
I now have to finish installing my software under X64.
Thanks for all your help.
DualBootPRO