After re-thinking my last post, here are two routes.

I think you can try either one of these paths. I am running two versions of Office and have several Themes folders.
Not sure if Windows sets that up or if Office sets that up (it is probably Office but depending on what version of Windows you're running it does make a difference).

In any event, these are the common paths:

Either - C:\Program Files\Common Files\Microsoft Shared\Themes

or - C:\Documents and Settings\[Your log in name]\Application Data\Microsoft\Themes

It should be one of these.

Scott