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My apologies Rojash and Chuck. I did reply to your comments very quickly, but the answer hasn't appeared so I guess the connection was interrupted yet again (BT - Big Trouble).
I was certainly trying to open from Documents, Rojash, so it wasn't that. However.... egg on my face again. When I followed Chuck's suggestion and checked All Programs, I discovered that Powerpoint wasn't there. It used to be, but I must have deleted it for some reason.
Anyway, I installed the Powerpoint reader - or thought I had - but it hasn't appeared yet in the list so I may try again.
Sorry to have wasted your time yet again on a problem for which I should have been able to work out the solution myself.