Hi Dani,
Assuming you are running Windows XP not Windows Vista:
Firstly run this tool to re-install ASP.NET into IIS
c:\windows\Microsoft.NET\Framework\v2.0.50727\aspnet_regiis -u
c:\windows\Microsoft.NET\Framework\v2.0.50727\aspnet_regiis -i
If that does not work then re-create the ASP_NET user account
c:\windows\Microsoft.NET\Framework\v2.0.50727\aspnet_regiis -ga ASPNET
If this does not work read this page:
http://www.developersdex.com/asp/message.asp?p=592&r=5837738
If that does not work download WFetch from Microsoft and let us know what error you get:
http://support.microsoft.com/kb/284285