Sounds like a bad legacy third party add-in. Test by running in Safe Mode.
From Start, Run (Winkey+R), type in
winword /a
and press enter. Open a document and test for editing now. If it is OK now,
then you have installed something that has placed an incompatible add-in in
Word/Office.
Note that uninstalling/reinstalling is nearly always a waste of time because
user customisations, registry entries, macros, global and custom templates,
anti virus and printer drivers are all unchanged.