Here’s a little problem you might run into when trying to use aspnet_regiis.exe to register ASP.NET 4 on a site (pic 1). When you look into the mentioned log file and search for “fail” you find this (pic 2). So what’s that all about? “Turns out it’s actually down to the fact that aspnet_regiis.exe doesn’t cope too well with the generally accepted environment variable that expands to Windows directory. So when you look in your applicationHost.config file (in \windows\system32\inetsrv\config) you may find something like (pic 3). If you change the highlighted bits to the appropriate hard coded location (e.g. c:\windows ) and try again you should find it works.