This should work for you:
Select Top 1000000 tblAssets.AssetID As [Asset number],
tblAssets.AssetName,
tsysAssetTypes.AssetTypename,
tsysAssetTypes.AssetTypeIcon10 As icon,
tblMonitor.MonitorManufacturer,
tblMonitor.MonitorModel,
tblMonitor.SerialNumber,
tblAssets.Lastseen,
tblAssets.Lasttried
From tblAssets
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Inner Join tsysAssetTypes On tsysAssetTypes.AssetType = tblAssets.Assettype
Inner Join tblMonitor On tblAssets.AssetID = tblMonitor.AssetID
Where tblAssetCustom.State = 1
Order By [Asset number]