Try this:
SELECT TOP 100 PERCENT
dbo.tblComputers.Computername AS Computer,
dbo.tblDiskdrives.Caption,
CAST(CAST(dbo.tblDiskdrives.Freespace AS bigint) / 1024 / 1024 AS numeric) AS Free,
CAST(CAST(dbo.tblDiskdrives.Size AS bigint) / 1024 / 1024 AS numeric) AS [Total size],
dbo.tblDiskdrives.Lastchanged AS [Last changed],
dbo.tblDiskdrives.Size
FROM
dbo.tblComputers
INNER JOIN dbo.tblDiskdrives ON (dbo.tblComputers.Computername = dbo.tblDiskdrives.Computername)
INNER JOIN dbo.tblOperatingsystem ON (dbo.tblComputers.Computername = dbo.tblOperatingsystem.Computername)
INNER JOIN dbo.Web30repAllServers ON (dbo.tblComputers.Computername = dbo.Web30repAllServers.Computername)