Have you downloaded any updates for Office? Do a search of your computer for a folder called msocache. It should be hidden. In there should be your MSI files. I don't know if you cvan get replacements off the web, but you may be able to borow a friend's copy of office XP and use it tog et them.
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If your equation requires either a complex combination of variables to get the desired result OR you can't seem to get the result regardless of the variables....it ain't your variables, it's your constant.