I have used aspnet_regiis for some time but I learned something new the other day. After installing the .NET 4.0 framework on a Windows 2003 Server box and running aspnet_regiis –i –enable I found that it changed all the sites to use the 4.0 framework. Not an issue if I wanted all of them to run under .NET 4.0 but I only wanted one to run under the 4.0 framework so I had to change the others back to use the 2.0 framework.