Yes you could use ASP for this. Even then a couple of ways are possible.
Conditional includes are discussed in the topic:
You could use a select statement to determine the file you wish to include, use the file system object and use some string manipulation to write the BODY of the page to the browser.
XML: There is probably a more elegant way to do this with XML if your prepared to have a look around. It think the object to use is Microsoft.XMLHTTP.
They say, best men are moulded out of faults,
And, for the most, become much more the better
For being a little bad.