The sleep APi basically puts the PC to sleep. So ... It probably won't see the click on the button.
SetTimer is the way to do this but you really need to know what you are doing/ SetTimer calls a macro memory location every xxxx micro seconds. This will do what you need BUT if anything is wrong and the called macro is not at that locatiion or throws an error you will totally crash the PC. Shyam has sample code on the same page but be careful! If you are able to adapt it so KillTimer is always called on error try it ... Don't blame me (or Shyam) when it crashes though!
You should make sure the Timer is killed when the slide show ends
You could add this in the TimerProc
If SlideShowWindows.Count < 1 Then
bTimerState = True
Call TimerOnOff
Exit Sub
End If
It IS not completely foolproof though