SSI on Apache vs IIS/4.0

I need some help figuring out why my use of SSIs that worked fine on apache isn't doing anything now that my site has been moved to a IIS/4.0 server. Technically, I guess that's not an apache question, but I'm hoping there is some obvious issue that people more familiar with apache would know about.

Here's what's going on:

While
<!--#echo var="QUERY_STRING"-->
works, displaying the query string, and

<!--#include virtual="foo.htm" -->
works, including the file...

<!--#include virtual="$QUERY_STRING" -->
doesn't work, and I can't figure out why. Again, this has worked for me on apache. An issue with environment variables that I don't know about??

Any help to a neophyte, including stating the obvious, would be much appreciated.

--Steve