This is quite a common problem it seems - after installing an update to Microsoft office, some of the applications will not open.

Here is one solution:

1. Create a temporary Windows login account with administrative rights, then log into that new account.
2. While logged in as the temporary user, open every Office product you have, then close one at a time (Excel, Word, Powerpoint, Project etc...)
3. After opening every Office application, start regedit. (Start > Run > then type in Regedit)
4. Go to HKEY_CURRENT_USER > Software > Microsoft > Office, and highlight Office
5. Right click on Office, and select 'Export'
6. Save the .reg file somewhere on your computer - note down the location
7. Log off the temporary account, and login under your normal Windows account (the one giving you the error)
8. Open regedit again, and then go to File > Import and pick the file that was exported.
9. After the import completes, the applications should open correctly.