You need to edit the view with the report builder and change it to :
SELECT TOP 100 PERCENT dbo.tblerrors.Computername AS Computer, dbo.tblerrors.ErrorText, dbo.tblADComputers.Description,
MAX(dbo.tblerrors.Lastchanged) AS [Last changed]
FROM dbo.tblerrors LEFT OUTER JOIN
dbo.tblADComputers ON dbo.tblerrors.Computername = dbo.tblADComputers.Computername
GROUP BY dbo.tblerrors.Computername, dbo.tblerrors.ErrorText, dbo.tblerrors.CFGname, dbo.tblADComputers.Description
HAVING (dbo.tblerrors.ErrorText LIKE '%RPC server is unavailable%') AND (dbo.tblerrors.CFGname = 'WMIERROR')
ORDER BY MAX(dbo.tblerrors.Lastchanged) DESC