Since you mentioned the includes folder, sounds as if the .asp is being used to simply "include" (I know, redundant) information on various pages from a central point. This is actually very convenient for updating your site as you can do it from a central location rather than page by page.
I suspect you should see in the raw html view in an editor (meaning not 'view source' on a rendered page) tags like
<!--#include file="header.htm" --> or
<!--#include virtual="/header.htm" --> (Personally, I prefer this method)
This would clearly indicate that, say header information, is edited in the header.htm page, possibly located in your includes folder. You might also have one for the footer, navigation, and so on. The include file should use normal html markup, but generally should not have a <doctype> <html> <header> or <body> tag, that would be in the page the include is located in.
The down side of some includes, especially navigation, is the links. Doing <a href="page.htm"> or <a href="../page.htm"> could cause a broken link depending on the depth of the page. Again, I personally use <a href="/page.htm">, or if it is in a folder,
<a href="/folder/page.htm">
Bottom line, for the purpose I suspect that .asp is being used, I do not thing you will have a problem once you get the hang of it. My suggestion is set up some test pages with the necessary includes and experiment. Take it from me, making an error in a test page is a lot better than doing one that will affect your entire site.
Marshall
