Font changes when editing

When I edit a file imported from a PC with XPsp3 the text in preceding paragraph usually - not always - changes from Times New Roman 12pt to Calibri 11pt. I can revert it ok, but how can I avoid it happening in the first place?

(Ver 4.3.7.2/Win7. Tried upgrading to 4.4.5 but download always fails)

Thanks for any help.

Peter