Normally when you see lines of any type that shouldn't normally be there, it's down to a fault with either the monitor or graphics card. However, I'd rule the graphics card out in this case as normally when you see lines on startup with a faulty GPU, Windows will start but your screen would be completely blank so I'd say that you've either got a slightly faulty display or an odd windows glitch. It could, of course, be down to several other things, but it's best to start with the simple and potentially obvious ones.
Here are a couple of things to try:
1) If you can, connect your laptop to an external display (such as a Full-HD TV or monitor) and boot it up; if you DON'T see the lines, it's likely to be a fault with your laptop's screen and it would be best to RMA it. If you DO see the lines it could either be the beginnings of a fault with your GPU or just an odd Windows glitch.
2) Try playing about with the resolution of your screen and switch down to the lowest it can go and progressively work through the resolutions until you're back at 1920x1080 and see if the lines appear on any of the lower resolutions - if they do I'd definitely RMA it as it could get worse over time.
Small caveat: I googled the issue you're having and nearly all of the results that come back were down to faulty graphics cards. The other guys on here might have some other suggestions, but it wouldn't do any harm to get in touch with Asus in the meantime to see what they say as they'll run you through some extra troubleshooting.
Asus ROG G751JT-T7115H
17.3" IPS, Intel i7-4720HQ, 16Gb RAM, NVidia GeForce GTX970m, 256Gb M.2 SSD + 1Tb SSHD