Custom 404s and Spanish-language characters

Hi everyone,

I'm trying to redesign a website here:

http://www.libreriapandevida.com/newsite/

The problem is that I'm trying to implement a custom 404, and
Spanish-language characters are posing a problem. If you look down the
middle of the page, and click the 4th product (Vives en m=ED -Pista),
you'll see the error:

The system cannot find the file specified.

http://www.libreriapandevida.com/newsite/pistas/vives_en_m=ED_-pista_760.htm

For those who want the direct link.

I know this has to do with the Spanish character, because if I replace
it with something else it works fine. The problem is that the Spanish
character is necessary since this is how it's stored in the database
and I don't want to do something like this:

http://www.libreriapandevida.com/newsite/libros/adicci--243--n_a_la_aprobac=
i--243--n_761.htm

to solve the problem.

So...what would I have to do, if anything, to configure my custom 404
to work within IIS to accept Spanish-language characters?

Thanks.