If you try to install a .NET update, MS08-052 for example, you may see an event log that looks like:

Event Source: ASP.NET 2.0.50727.0
Event ID: 1018
·   Failed while registering ASP.NET (version 2.0.50727.0)
in IIS. Error code: 0x80070003

This message is informing you that ASP.NET could not register with IIS.  The most common reason for this is that IIS (w3svc) is not currently installed.  For example, if you just have FTP installed, you will get this error when installing this update.

The problem comes in when aspnet_regiis attempts to run during install.  This event is not anything to worry about as it is just informing you that it could not install.  If IIS (w3svc) is installed and you get this error, then it is something you need to look into.  Either check the install log or run aspnet_regiis again from a command prompt to see what happens.  Be sure to check the help so you use the correct switches.