Hi 
try this, so this will exclude any pcs with names containing PC12 , PC34 and PC56
Select Top 1000000 tsysOS.Image As icon,
  tblAssets.AssetID,
  tblAssets.AssetName,
  tblAssets.Domain,
  tblAssets.Username,
  tblAssets.Userdomain,
  tblAssets.IPAddress,
  tsysIPLocations.IPLocation,
  tblAssets.Firstseen,
  tblAssets.Lastseen,
  tblAssets.Lasttried
From tblAssets
  Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
  Inner Join tsysOS On tsysOS.OScode = tblAssets.OScode
  Inner Join tsysIPLocations On tsysIPLocations.LocationID =
    tblAssets.LocationID
Where tblAssets.AssetID Not In (Select Top 1000000 tblSoftware.AssetID
      From tblSoftware Inner Join tblSoftwareUni On tblSoftwareUni.SoftID =
          tblSoftware.softID
      Where tblSoftwareUni.softwareName Like 'Trend%' And
        tblAssets.AssetName Not Like '%PC12%' And tblAssets.AssetName Not Like
        '%PC34%' And tblAssets.AssetName Not Like '%PC56%') And
  tblAssetCustom.State = 1
Order By tblAssets.Domain,
  tblAssets.AssetName