Why won't Windows 7 install my downloaded .NET 3.5 SP1?

"Make sure you're installing from an account that has administrator priveleges.

Also, if you're on a domain, make sure there aren't any group policy restrictions in place. I've encountered the error you're getting when attempting to install Visual Studio as a user with local admin rights, because there was a group policy restriction preventing access to the security log (which the .NET installer needed access to, for some reason)."