The ForceLogOff works in that it forces the user off of the Server connection as it was intended.

Here is a link that discusses it more.
http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/566.mspx

As for the scheduled task not being created, as you have it shown it needs a password which you're not supplying. You can't use /U for the SYSTEM account, you need to use /RU

If that does not correct the issue please let me know, but this works for me, however I don't think you have the time set correctly either as it wants to pretty much run as soon as it is created.

Code:
Break On

Dim $SO

$SO=SetOption('Explicit','On')

$SO=SetOption('NoVarsInStrings','On')

 

Dim $MB

$MB=MessageBox("Configuring Automatic Logoff...","Network Security",64,2)

Shell '%COMSPEC% /C schtasks /delete /tn AutoLogoff /f >nul 2>nul'

Shell '%COMSPEC% /C schtasks /create /sc minute /mo 30 /ru System /tn AutoLogoff /tr \\utopia\netlogon\autologoff.bat >nul 2>nul'