When I attempt to save an already saved and opened doc, I get the following error message:

Word cannot complete the save due to a file permission error. (D:\...|Doc name)

I would appreciate someone letting me know why this may be, so I can avoid it in the future (I've had it before - my usual way round this is just to rename the file by adding a letter or number to the existing name).

The problem may be that the Temp folder used by Word is overcrowded. To clear it, input:
C:\Users\%Username%\AppData\Local\Temp
into Windows Explorer, then delete all files/folders there. Ideally, you'd do this after closing all other programs; otherwise Windows Explorer will complain it can't delete some files/olders (you can skip those).

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]

Thanks Macropod. That worked!