When you use the Resource Kit Remote Console to perform an unattended install of SP5 or SP6a:

update.exe /u /q

runs, but doesn't do anything usefull. If you use Task Manager, you see a Service Pack Setup Error.

The problem is caused by the failure of the Remote Console client to set the %USERPROFILE% environment variable, as it loads the users NTUSER.DAT.

Setup doesn't accually need the correct user, so running:

set userprofile=%systemroot%\profiles\default user
update.exe /u /q

works just fine.

NOTE: See tip 1981.