Make sure you boot into BIOS (hit the key that is listed as Setup, probably F12, right after turning it on) and check what the computer is detecting as your screen. Often, it ends up refreshing detected hardware, and behaves normal.
If anything, it must be Windows setting the resolution wrong. You can go into Control Panel and select Appearance for whichever version of Windows you have. From there, you can just select the largest possible resolution and see what happens when it adjusts.