This is totally happening to me too! I thought I had a virus or something but no! I reinstalled office (ripped it out totally), renamed my Normal.Dot file and nothing. I verified I have all the current service packs, which I suspect may be the problem. Possibly a windows update issue as well.
I have found no solution, it happens really randomly. I can't even paste within the same document, it doesn't keep the formatting, just "keep as text only". Frustrating!