If you've logged onto the properties for your IIS install and found that the ASP.NET tab has mysteriously disappered, you can try a couple of things.
Firstly, try re-registering ASP.NET with IIS using the ASPNET_REGIIS.exe located in the .NET installation folder:
Chances are though, that it won't work, and that you can try and number of command using aspnet_regiis.exe or even uninstalling and reinstalling .NET and you won't actually fix the problem. (Note: on x64 systems this could be a different problem, Google again with x64!)
This was solved by switching a flag (Enable32BitAppOnWin64) to false in the IIS metabase:
cscript.exe adsutil.vbs set W3SVC/AppPools/Enable32BitAppOnWin64 FALSE
You should then be able to see the ASP.NET tab restored in the site properties.