What's the current key for "FileSaveAs" command in the "Customize Keyboard"?
Does the F12 work before?
Please rename the normal.dot file, and launch the word again, see if the
issue still appears. You can refer to the renaming steps in the KB 826862
http://support.microsoft.com/kb/826862
Resources:
Resolve unexpected Function (F1 - F12) or other special key behavior
http://support.microsoft.com/kb/894513