"When I Paste Special and paste it as Unformated text it works fine."
As to the cause, that's a Microsoft programming issue and given the age of this old software the "Why?" may never find an answer. The paste special is there for a reason.
When I copy a block of text from one Word Document, and Paste it into another the font changes to Arial Black.
The style of the block of text is "Normal" the Normal Style in the origanal document and the Normal.dot template that I am pasting the text into are both set to Arial.
I have tried using a different Normal.dot (from a friends PC) and the same happens. I have tried re-saving the Normal.dot with the default font updated, and it still happens.
On my friends PC they can not recreate the error even using the same origanal files (although it happens whenever I do this) and normal.dot.
When I Paste Special and paste it as Unformated text it works fine.
Any ideas on what is causing this?