Hello crickets,
Since you're already going thru a conversion process, you should convert to 2008! There's a big difference and it'll make your life easier down the road. But assuming you can't....
The most common reason you get these error is when inherits attribute in you .aspx page directive is not the same as the class name in your code behind.
<%@ Page Language="C#" CodeFile="Default.aspx.cs" Inherits="aaaaaaa" %>
aaaaaaa must be the name of the class in the code behind. If you have a namespace in the code behind, the inherits attibute must be "the.namespace.aaaaaaa"
~Rick