the key table/View is vOSOperationSystem or Inv_OSOperating_System. It has the Last Boot Up time.
This can be found in Resource Manager for a computer
Try this query... Throw this query in a Altiris Report.
select
vc.name as [Computer Name]
,vc.[OS Name]
,vc.[User]
--,b.[Last Boot Up Time]
,ISNULL(CAST(b.[Last Boot Up Time] AS NVARCHAR), '') as 'Last Boot Up Time'
from vComputer vc
join vOSOperatingSystem b on b._ResourceGuid = vc.Guid
order by ISNULL(CAST(b.[Last Boot Up Time] AS NVARCHAR), '') desc