Should be something like
SELECT dbo.tblComputers.Computername, dbo.tblOperatingsystem.ServicePackMajorVersion, dbo.tblOperatingsystem.Version
FROM dbo.tblComputers INNER JOIN
dbo.tblOperatingsystem ON dbo.tblComputers.Computername = dbo.tblOperatingsystem.Computername
WHERE (dbo.tblOperatingsystem.ServicePackMajorVersion = '2') AND (dbo.tblOperatingsystem.Version = '5.1.2600')