I'm with Simonta on this one, because the original post suggests that the screen is fine up to login.
This suggests to me that there is a problematic driver that gets loaded only when a user logs in, or there is some sort of user setting being applied at log in.
I would be examining all the control panels for graphics card and monitor settings, and also looking at the power setup, to see if any energy saving settings are kicking in.
Often laptops have some third party control applications installed for setting up the graphics card, those would need checking too.