I hadnt tested it yet, having done so now I think you need #Persistent in there to keep the script from ending (and dropping the timers). Also, I added the timer to timeout and made it just give the message box, that trips the checkwindowstate and lets your screensaver activate without being a totally annoyance on the off chance that you're still watching

.
I think this is a keeper, anyone? Again big thanks to TheIrishThug for making this much more sensible & tidy.
Update: Tested in XP & Windows 7; success!
Solved previous issues of not detecting netflix's fullscreen mode and of not preventing monitor power down/sleep/standby mode. The fix for the 2nd issue comes from
here. The fix for the first issue is admittedly still noob-ish since it simply looks for a "AGFullScreenWinClass" window concurrent with the netflix window and I doubt that ahk_class is unique to netflix. Unfortunately the fullscreen window is titleless. I'm certain there's a way to grab the unique ID of the AGFullScreenWinClass window opened by netflix, but I left it as is for the sake of simplicity & performance (ok, and a bit of laziness too) ...still it seems "false positives" would be very rare.
Anyway, enjoy!
Code:
#NoEnv ; Recommended for performance and compatibility with future AutoHotkey releases.
#Persistent
ES_CONTINUOUS := 0x80000000
ES_DISPLAY_REQUIRED := 0x00000002
windowsState := 0
sleepDelay := 150 ; mins
Menu, Tray, Icon, SHELL32.dll, 44
SetTimer, CheckWindowsState, 1000
OnExit, ExitSub
CheckWindowsState:
If ((!windowsState && WinActive("Netflix: Netflix Movie Viewer")) || (!windowsState && WinExist("Netflix: Netflix Movie Viewer") && WinActive("ahk_class AGFullScreenWinClass")))
{
windowsState := 1
Menu, Tray, Icon, user32.dll, 2
TrayTip, Netfix, Screen Saver Disabled`, Enjoy The Show!, 10, 2
DllCall("SystemParametersInfo", Int,17, Int,0, UInt,NULL, Int,2) ;disable screensaver
DllCall("SetThreadExecutionState", UInt, ES_CONTINUOUS + ES_DISPLAY_REQUIRED) ;prevent sleep
SetTimer, NetflixTimeOut, % -sleepDelay * 60*1000 ;run only once
}
Else If ((windowsState && !WinActive("Netflix: Netflix Movie Viewer")) || (windowsState && !WinExist("Netflix: Netflix Movie Viewer")))
{
SetTimer, ScreenSaveActivate, -1 ; run sub and turn off the timer
}
Return
ScreenSaveActivate:
Menu, Tray, Icon, SHELL32.dll, 44
TrayTip, Netfix, Screen Saver Enabled!, 10, 1
DllCall("SystemParametersInfo", Int,17, Int,1, UInt,NULL, Int,2) ;enable screensaver
DllCall("SetThreadExecutionState", UInt, ES_CONTINUOUS) ;enable sleep
SetTimer, NetflixTimeOut, OFF
windowsState := 0
Return
NetflixTimeOut:
MsgBox, Zzzzzzzz...
Return
ExitSub:
Gosub, ScreenSaveActivate
ExitApp