I have Vista computers that SEPM cannot find either by IP or by computer name. All of the have Network Discovery turn on, File Sharing turned off, and the account has elevated privileges on the computer. Some have SAV on them others have nothing as I un-installed SAV. I am not sure if the problem is that our SAV server, which will be decommisssioned soon, is causing this but it needs to remain on-line until each computer has been migrated over. Note: XP machines can be found, although some of them will not show the SEP as being installed; SAV shows as being installed still.
I have been working on this issue for 4 weeks now. I am not able to do a manual install at this point because the 'company' specific customizations for SEP do not take effect and the end user can do anything they want.