I looked through similar entries for this problem but none of the ones I found involved the "InstallWizard.aspx" file, so I apologize in advance if someone has already posted about this. 

I've been trying to install DNN on my own machine to see if it works before putting it on the server. I'm using SQL Server Express 2005. I've gone through all the steps listed in the installation guide and I can't get past this one. I type in the URL http://localhost/DotNetNuke/Install/Install.aspx?mode=Install, which is supposedly the final installation step, and this is what I get:

Object reference not set to an instance of an object.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

Line 306:
Line 307:            'Don't process some of the AppStart methods if we are installing
Line 308:            If Not HttpContext.Current.Request.Url.LocalPath.EndsWith("InstallWizard.aspx") Then
Line 309:                'Check whether the current App Version is the same as the DB Version
Line 310:                CheckVersion()

[NullReferenceException: Object reference not set to an instance of an object.]
   System.Web.Hosting.ISAPIWorkerRequestInProc.GetServerVariable(String name) +1126
   System.Web.Hosting.ISAPIWorkerRequest.ReadRequestHeaders() +121
   System.Web.Hosting.ISAPIWorkerRequest.GetKnownRequestHeader(Int32 index) +126
   System.Web.Hosting.ISAPIWorkerRequestInProc.GetKnownRequestHeader(Int32 index) +104
   System.Web.HttpWorkerRequest.HasEntityBody() +17
   System.Web.HttpRequest.GetEncodingFromHeaders() +223
   System.Web.HttpRequest.get_ContentEncoding() +48
   System.Web.HttpRequest.get_QueryStringEncoding() +7
   System.Web.HttpRequest.get_QueryStringText() +3310420
   System.Web.HttpRequest.get_Url() +43
   DotNetNuke.Common.Global.Application_Start(Object Sender, EventArgs E) in C:\DotNetNuke\App_Code\Global.asax.vb:308

Any help is greatly appreciated.

When I just use http://localhost/DotNetNuke the same thing happens, I get the same exact error screen.