This thread should give you a good example : http://www.lansweeper.com/forum/yaf_postst1159_Query-for-Computers-with-No-Antivirus-Installed.aspx
should be something like this:
Select computername from tblcomputers where computername not in (SELECT ComputerName FROM dbo.tblSoftware WHERE softwareName LIKE 'Sophos Anti-Virus')