Just filter on the time generated:
Select Top 1000000 tblAssets.AssetID,
tblAssets.AssetName,
tblOperatingsystem.Caption,
tblAssetCustom.Manufacturer,
tblNtlogSource.Sourcename,
tblNtlog.Eventcode,
tblNtlogMessage.Message,
tblNtlogFile.Logfile,
tblNtlog.TimeGenerated
From tblAssets
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Inner Join tblNtlog On tblAssets.AssetID = tblNtlog.AssetID
Inner Join tblNtlogSource On tblNtlogSource.SourcenameID =
tblNtlog.SourcenameID
Inner Join tblNtlogMessage On tblNtlogMessage.MessageID = tblNtlog.MessageID
Inner Join tblNtlogFile On tblNtlogFile.LogfileID = tblNtlog.LogfileID
Inner Join tblOperatingsystem
On tblAssets.AssetID = tblOperatingsystem.AssetID
Where tblNtlog.TimeGenerated > GetDate() - 7 And tblAssetCustom.State = 1