Please try this:
Select Top 1000000 tblAssets.AssetID,
tblAssets.AssetName,
tsysAssetTypes.AssetTypename,
tsysAssetTypes.AssetTypeIcon10 As icon,
tblUsersInGroup.Groupname,
tblUsersInGroup.Domainname,
tblUsersInGroup.Username,
tblUsersInGroup.Admingroup,
tblAssets.Lastseen
From tblAssets
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Inner Join tsysAssetTypes On tsysAssetTypes.AssetType = tblAssets.Assettype
Inner Join tblUsersInGroup On tblAssets.AssetID = tblUsersInGroup.AssetID
Where tblUsersInGroup.Groupname = 'Power Users' And tblAssetCustom.State = 1