I know this works with the shell function but I'm not sure about the API calls. Maybe this might help you figure it out. When you do can you post it?

'Control Panel itself:
X = Shell("rundll32.exe shell32.dll,Control_RunDLL")

'Display:
X = Shell("rundll32.exe shell32.dll,Control_RunDLL access.cpl,,3")

'Background:
X = Shell("rundll32.exe shell32.dll,Control_RunDLL desk.cpl,,0")

'Apperance:
X = Shell("rundll32.exe shell32.dll,Control_RunDLL desk.cpl,,2")

'Screen Saver:
X = Shell("rundll32.exe shell32.dll,Control_RunDLL desk.cpl,,1")

'Settings:
X = Shell("rundll32.exe shell32.dll,Control_RunDLL desk.cpl,,3")

'Date/Time:
X = Shell("rundll32.exe shell32.dll,Control_RunDLL timedate.cpl")