"peek-a-boo" processes preventing entering System Standby

I am having trouble with a Windows XP SP3 - Media Center Edition system not entering System Standby.

On observing Task Manager, there appears to be 3 processes under the SYSTEM user that may be aggravating the problem: winlogon.exe, logonui.exe, and lsass.exe. They appear briefly approximately once every 30 seconds or so.

This system runs Norton 360 Premier Edition in the background; could it be the source of these processes? Are these processes associated with paging? Could some other background process be the culprit?

Thanks in advance for any insight you might be able to provide.