I don't have any way of testing this, but it should work..........
SELECT Software.ProductKey, Software.ComputerID, Count(*) as PCount
FROM Software, Computer
WHERE Software.ComputerID = Computer.ComputerID
AND Software.Version ='2000'
AND Software.SoftwareName = 'Microsoft Office'
GROUP BY Software.ProductKey, Software.ComputerID
HAVING PCount >1;
Including the ComputerID in the mix may hide some of the results if you leave the HAVING clause in the SQL.