can't preview ASP pages

i have a strange problem. It started with only one site, but now the problem
extended to every site i have in my dreamweaver MX. I cant preview ASP files
(just ASP, html works fine), I get a 500 Internal Server Error, but the
thing worked just fine just a couple of days ago. At first i had the problem
with a new site, but then i reinstalled IIS and Dreamweaver MX, and not only
the problem persisted, but it extended to every site.

I have Win XP pro SP1 in spanish. (Dreamweaver in english)

please help!!!
thanks in advance,

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Nacho