Trying to create Shell commands on a single line is bad form.. Using something like this makes it more managable and can provide a debugging message as well:
 Code:
$Cmd = '%COMSPEC% /c ' ; note trailing SPACE
$Cmd = $Cmd + '"\\' + $LDSERVER + '\LDLOGON\PATCH\' + $LANDESKDBRSVULPATCH + '" '
$Cmd = $Cmd + 'INSTALLCMD="/notip /noqp /autoinstall" ADDDESKTOPICON=0 '
$Cmd = $Cmd + 'ADDSTARTMENU=0 DEFCOMPICK=LEGACY SHOW_WEBPAGE=0 USAGEPICK=USAGE_NO /QR'
'DEBUG - About to run: ' @CRLF $Cmd @CRLF
Shell $Cmd
'DEBUG - Result: ' @SERROR @CRLF
Of course, this is only half of your logic, but you can see that each line contains only one embedded double-quote pair. This makes it easier to keep track of open/close quotes. Using the SANITY UDF or my KGen tool will help as well. Also note that each line except the last contains a space.

Glenn
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Actually I am a Rocket Scientist! \:D