We have mentioned countless times in this book (or so it seems) that you should not install anything on your UAG server, other than UAG itself. This, of course, raises the question of an Antivirus. Having good virus protection is very important, and the "do not install" statements are not referring to this, although it does require some additional considerations.
An Antivirus scanner, by design, may have to scan all system files on a server, and it may "hold" a file in use when it does so. UAG, TMG, IIS, and SQL running on the server may not appreciate this and behave strangely. To guarantee stable performance, you should exclude certain folders and files from the Antivirus scanning process.
Excluding paths and files is different for every Antvirus product, so consult your documentation on how to do this. The paths that need to be excluded are:
%ProgramFiles%\Microsoft Forefront Threat Management Gateway
%ProgramFiles%\Microsoft SQL Server\MSSQL10.ISARS...