UPDATE:

I created a Default.htm file and placed it in the directory \DNN_451_Source, it loaded OK. This causes me to believe that IIS and name resolution are working OK.

I then deleted Default.htm and tried again. Internal Error 500, as before. I've noticed that if I point at http://DNN_451_Source, I get the Internal Error 500. If I point explicitly at http://DNN_451_Source/Default.aspx, then I get redirected to the Google Search page (default search on this machine)

I downloaded WFETCH from the MS site, and when I run WFetch, the response I get is as follows:

WWWConnect::Connect("localhost","80")\nIP = "127.0.0.1:80"\nsource port: 1559\r\n
REQUEST: **************\nGET /DNN_451_Source HTTP/1.1\r\n
Host: localhost\r\n
Accept: */*\r\n
\r\n
RESPONSE: **************\nHTTP/1.1 302 Object Moved\r\n
Transfer-Encoding: chunked\r\n
Location: http://localhost/DNN_451_Source/\r\n" target="_blank" rel="nofollow">http://localhost/DNN_451_Source/\r\n
Server: Microsoft-IIS/5.1\r\n
Content-Type: text/html\r\n
--------------: ---\r\n
\r\n
\n

Object Moved

I now pointed WFetch at http://localhost/DNN_451_Source/Default.aspx, and I get the following results:

started....WWWConnect::Connect("localhost","80")\nIP = "127.0.0.1:80"\nsource port: 1567\r\n
REQUEST: **************\nGET /DNN_451_Source/Default.aspx HTTP/1.1\r\n
Host: localhost\r\n
Accept: */*\r\n
\r\n
RESPONSE: **************\nHTTP/1.1 500 Server Error\r\n
Transfer-Encoding: chunked\r\n
Server: Microsoft-IIS/5.1\r\n
Date: Sun, 20 May 2007 13:49:30 GMT\r\n
Connection: close\r\n
Content-Type: text/html\r\n
--------------: ---\r\n
\r\n
\n

Server Application Error

I'm using SQL Server Express and Visual Stusio STANDARD (not Express)

The Default.aspx was originally not listed in the Documents tab, but I added it after setting up the virtual directory. It is also at the top of the list.

Actually the tutroial I used was How to install DotNetNuke 4.5 Source Code to Windows XP Professional

Are you suggesting that I do a "runtime" install first, prior to source code install?