I would be interested too if anyone has a COM object out there.
Otherwise I would suggest you write a perl ASP or CGI page. Perl can
fork out to external executables and there are modules for handling
SMTP internally, which you need because there's no standard equivalent
to /usr/lib/sendmail on NT.
There's no particular reason to write it as an ASP instead of a CGI.
People will undoubtedly claim there's a performance advantage, but
that's completely irrelevant if you're doing something relatively slow
like signing or encrypting a message. (Even on NT where fork is
excruciatingly slow: the real reasons threads are so popular on NT.)