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However, you might consider simply using mod_rewrite to do an internal rewrite from .htm to .php, and leaving your current URLs alone. It's just as easy as redirecting, and has the advantage of not affecting your search listings and visitors' existing bookmarks. Using an internal rewrite, you change the relationship between the publically-visible URL (.htm) and the server filename (.php) that that URL refers to. For an example, you have to look no further than the address bar in your browser right now... This site is dynamic, but presents static URLs for most functions.
As much as I dislike meta-refreshes and all the trouble they have caused in the search engines recently, that may indeed be your best bet on a shared Windows hosting account.