I believe this is happening because createobject in kixtart does not support a second parameter. You should be able to pull this off by using the scriptcontrol object.

Something like...

(untested)
 Code:
$sc = CreateObject("ScriptControl")
$sc.language = "VBScript"
$sc.addcode('Session=CreateObject("Microsoft.Update.Session","' + $strWks + '"')
$sc.run
$ObjSession=$sc.codeobject.session