Beware of the security update for .NET 1.1 - KB886903 . I've just spent several hours trying to work out why my machine running VS.NET 2003 and .NET 1.1 on Windows XP Pro SP2 suddenly started to return the following error every time I tried to launch a site in debug mode from Visual Studio: Error while trying to run project: Unable to start debugging on the Web server. Could not start ASP.NET or ATL Server debugging. Verify that ASP.NET or ATL Server is correctly installed on the server. If the site is launched without debugging then ASP.NET returns a 404 error page. Very strange. After much head scratching and trying various workarounds I uninstalled and reinstalled the .NET Framework 1.1, then installed SP1 for the Framework and, hey presto, everything started working again. However, Windows Update then prompted me to install the security update, which I did, restarted the machine and, lo and behold, all the errors were back again. That kind of gave it away. I uninstalled the KB886903 update and now it's all up and running. I'm yet to find a workaround for installing the update without breaking ASP.NET.