I, too, am having this issue. DNN 4.3.3. StarterKit, VS2005 on windows XP using IIS web and SQL server database, which are all working fine.  When I build the website, I get an error creating control on the page, and nothing loads. 

I notice that the code behind for the default.aspx page has an error notice underline on the dnn:form tag

dnn:Form id="Form" runat="server" ENCTYPE="multipart/form-data" style="”height: 100%;"

this tooltip text is:  "Element 'Form' is not a known element.  This can occur if there is a compilation error on the website."  Is it possible there is a missing dll reference?  or a date mismatch for the dlls?  I did notice there was an HTML error notice on the input tags inside the dnn:form tag, they are empty and require a "/>" instead of a ">" closing tag.  After adding those and rebuilding the page, it loads!  But, even though I do not think this really fixed it, I thought I'd share.  Perhaps it was the second build...