You can actually force IE (and some other browsers) to download instead of viewing inline for known file types.
You need to add a header like:
Content-disposition: attachment; filename=fname.ext
As per RFC 2183 http://www.nic.mil/ftp/rfc/rfc2183.txt
Famously, support for this is broken in IE 4.01, but like that matters these days (in 4.01 it's not honoured, but then you haven't lost anything).