MSN Homepage automatically open at startup

I'm an admin of a cyber cafe in my local area. All cient PC's (Windows 8.1 x64) are connected to each other including an Admin PC via wireless wifi. Recently, i change the router settings to hide its SSID. The only wifi available in Windows list are Hidden Wifi. I set and make each of the PC connect to the hidden wifi automatically on startup and it works normally. The only annoying problem, is a homepage MSN is automatically load at default browser whenever the hidden wifi is connected each time computer starts. It's pretty annoying as I need to ensure my client redirected to my desired homepage. I set IE, Chrome and Mozilla to specific website. But eventually each time computer boot, it automatically open a default browser as an MSN homepage. I can assure this annoying problem rises when it connect to hidden network and not occur when connect to normal wifi with SSID. I've searched everywhere on Google but no solution for this. So I'm asking in here instead. How to remove the MSN homepage automatically open at startup even though i set homepage as something else. Is there any way to prevent this?

P/S : And oh yes, don't tell me to check startup items in start shortcut, msconfig etc. I already check them and there are no reason for browser to open automatically at MSN homepage!