Go to `Control Panel/User Accounts` and see what accounts are there and what privileges they have. If you are logged on as Administrator you should be able to open MS Word, if you have created another account for yourself you may not have given yourself enough privileges or taken them away.
When you installed it did you say `for all users of this PC` or `only me`? The safe option is all users.
The only other option I can think of is to completely uninstall, delete all files from Programs and run Ccleaner, or similar, do a restart run Ccleaner again and then try a full installation of Word using the `all users` preference.