select one row based on a string
Below is an example using a CTE with conditional aggregation.
WITH inventory_per_computer AS (
    SELECT Computer_id, SUM(CASE soft WHEN '7Zip' THEN 1 ELSE 0 END) AS Installed
    FROM dbo.SoftwareInventory
    GROUP BY Computer_id
    )
SELECT Computer_id, CASE Installed WHEN 0 THEN 'no' ELSE '7Zip' END AS Software
FROM inventory_per_computer
ORDER BY Computer_id;