Can I check this in the registry, under the kay HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Streams\Desktop?
Could it be that the value for the registry name TaskbarWinXP can be evaluated to see if the task bar is enabled?
Something like this:
;Script Options
$SO=SETOPTION("Explicit", "ON")
$SO=SETOPTION("NoMacrosInStrings", "ON")
$SO=SETOPTION("NoVarsInStrings", "ON")
$SO=SETOPTION("WrapAtEOL", "ON")
BREAK ON

DIM $TaskbarWinXP

If @DOS = "5.1"
    ; $TaskbarWinXP = ReadValue('HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Streams\Desktop','TaskbarWinXP')
    $TaskbarWinXP = ReadValue('HKEY_USERS\'+@SID+'\Software\Microsoft\Windows\CurrentVersion\Explorer\Streams\Desktop','TaskbarWinXP')
    Select
    Case Left($TaskbarWinXP,18) = '0c0000000800000002'
        ? 'disabled'
    Case Left($TaskbarWinXP,18) = '0c0000000800000003'
        ? 'Enabled'
    Case 1
        ? 'This is just impossible, you will never get here'
    EndSelect
EndIf

Get $SO