Just posting this in case anyone else has a similar issue
Downloaded and installed free edition of Avast one on Windows 7 pro which has microsoft office 2000 installed. Looked around AV and happy with everything. Seemed to be an excellent replacement for Bitdefender which is now not free.
A few days ago I noticed that some Word documents were opening in read only mode. Thinking it was a fault with Word I reinstalled it but no better. I then thought that it might be a permission issue but after trying a few things still the same. I then noticed that the same word document from a different folder opened OK. After a long time searching on the web read that it might be AV related. Stopped Avast for 10 mintes and everything was fine and back to normal. After a process of elimination discovered that it was the Ransomware Protection that was causing the problem. I have partly disabled this and now all OK
Dave
