I found the HtmlAgilityPack for Asp.net 1.1 here http://www.codefluent.com/smourier/download/htmlagilitypack.zip in this zip there are several .cs files. To use this in Asp.net 1.1 just import in the aspx page using the assembly directive
http://msdn.microsoft.com/en-us/library/d864zc1k%28v=vs.71%29.aspx
Something like this...
<%@ Assembly Src="File1.cs" %>
<%@ Assembly Src="File2.cs" %>