Try this:
SELECT p.name,
SUM(CASE WHEN i.installed = '1' THEN 1 ELSE 0 END) AS counter
FROM vRM_Software_Bulletin_Item_ExactType AS p
LEFT JOIN vPMWindows_ComplianceReportsDrilldown AS i ON i.SoftwareBulletin = p.Guid
LEFT JOIN vcomputer AS c ON c.guid = i._ResourceGuid
WHERE i.ReleaseDate > '2017'
AND i.BulletinState = '1'
AND p.name = 'MS18-03-W10'
GROUP BY p.name, i.Installed
Minor nit - I like to make all the SQL reserved words all-caps so they stand out a little better. I also use "AS" when I'm specifying aliases, because it's just a good habit to get into.
Furthermore, you don't need
DISTINCT
if you're using
GROUP BY
.