Find out if the problem is limited to Word, MS Office or still wider (in principle, it could even be a keyboard problem, with the Home-key thinking it's a G; that's easy to check if you have another one around).
Ctrl-home goes to the top of the window I'm typing this in (ctrl-G doesn't do anything). Same for Notepad. Same for Excel (cell A1 is selected with ctrl-Home, ctrl-G gives Goto-dialog). Didn't try Wordpad.
In Word, home goes to the start of the line, while G types a simple G. That's an easy test also.
If the problem is limited to MS Office, try the brute force solution: run the Office Removal Wizard (on the Office CD) and reinstall. If the problem is limited to Word, first try to uninstall and reinstall only MS Word from the Office setup. These aren't very elegant solutions, but most likely they are effective.
Of course, it could be some nasty keyboard remapper having installed itself (or being installed by someone else hating you). Then the above solution obviously won't help.
Let us know how you solve this interesting problem.
Kees