Just a little more info:
There was a virus that affected the File|Save, File|SaveAs and File|Open.
http://support.microsoft.com/?kbid=308360
OFF2000: Error Messages Caused by Nimda Virus When You Open or Save
Files in Office Programs
maybe you got hit.
If this isn't it, do you get error messages?
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And you may want to try resetting that option on the toolbar.
Tools|Customize (just to see that dialog)
click on File
rightclick on SaveAs
select Reset
Test it out.