Using a Zeus 4.3 webserver, I needed to process .htm files via either SSI or PHP, for the sake of includes.

Unfortunately, adding the following to my .htaccess:

     AddHandler server-parse htm

Resulted in browsers prompting users to download the requested HTM file.

My investigation determined that the reason was this server-parsed content was returned with the following content-type header: "application/x-server-parsed-html".
The "application" part of this header value seems to cause browsers to assume this is a binary file, which must be downloaded.

The solution is to explicitly tell Zeus that this content type should be "text".

My adjusted htaccess looks like this:

AddType text/x-server-parsed-html htm
AddHandler server-parse htm

I owe a thanks to http://www.aboutworks.com/shokodei/diary/2006/2006_03_a.html and googles "translate this page" functionality!