IIS 5.1 Server Application Unavailable Error

I get this error sometimes when I change the default application in IIS.
When trying to browse my default.aspx page I get the big red font error "Server Application Unavailable".

I fix this issue by executing the following commands:
1) Give modify access to the "IIS_WPG"and "Internet Guest Account" users to your web directory. Try see if that works, if not continue with step2.

2) aspnet_regiis -i
3) iisreset

Microsoft Instructions:
To fix IIS mappings for ASP.NET, run the aspnet_regiis.exe utility:
1. Click Start, and then click Run.
2. In the Open text box, type cmd, and then press ENTER.
3. At the command prompt, type the following, and then press ENTER:
"%windir%\Microsoft.NET\Framework\version\aspnet_regiis.exe" -i
In this path, version represents the version number of the .NET Framework that you installed on your server. You must replace this placeholder with the actual version number when you type the command.

Then,

1. Click Start, and then click Run.
2. In the Open text box, type iisreset.

Done! Go check out your site it should be up and running.