Make sure that your current working drive and directory are real as some processes will object if they are not, and they may not be during login / runas activities.

Output the value of the command so that you can check it - the event log is a handy place to write it if your script runs hidden:
 Code:
GO "C:"
CD "C:\"

$sCommand='"'+%COMSPEC%
	+'" /c "'+%SYSTEMROOT%+'\system32\runnas.exe" /user:rds\xxxxx "'
	+%LOGONSERVER%+'\netlogon\bginfo\bginfo.exe" "'
	+%LOGONSERVER%+'\netlogon\bginfo\rds.bgi" /nolicprompt /silent /timer:0 /password:xxxx /env'
 
$=LogEvent(4,0,"About to execute: "+$sCommand,"","MyScript.kix")
Shell($sCommand)