If you receive a pre-requisite check error "ASP.Net 2.0 is not enabled" while installing MOM 2005 or SCOM 2007 even after making sure that you installed ASP.Net using Add-Remove programs then try running,

aspnet_regiis -i -enable at the command prompt.

You can find the executable under C:\windows\Microsoft.NET\Framework\v2.0.50727

If you are using a x64 system then you need to run the tool from Framework64 folder

C:\Windows\Microsoft.NET\Framework64\v2.0.50727

aspnet_regiis -i -enable

Additionally, you also need to make sure that the SCOM2007 setup on a Windows 2008 Server expects IIS6 Management Compatibility role service enabled.