klappis: Anyone know what Unlock DX Surfaces option do?
It's actually "Unlocked" and it has a good chance of messing up your graphics even if you have the Win98 compatibility mode set to avoid severe graphics issues in the first place. Don't turn it on. I'll explain it in more detail when I get home from work if you're curious about the technical details.
Edit: So, as for the details...
Nox uses DirectDraw, which uses so called "surfaces" (similar to bitmaps) on which images can be rendered. The screen contents are also accessible through a surface (called the primary surface).
Normally you don't have direct read/write access to the pixel data stored in a surface, but you can still perform some other operations on it, e.g. copying a rectangular portion from one surface to another. In order to get direct access, you first need to "lock" a surface, which provides you with a pointer to access the surface data. When you're done, you need to unlock the surface. It's not really an option to keep a surface permanently locked, since you cannot perform other important operations on a surface while it's locked.
According to DirectDraw documentation, it's also not valid to access the surface memory once it has been unlocked. But it seems that Nox devs didn't take this to heart, and by default (at least on Win 2000 and above or so) they only lock/unlock the primary surface (and its back buffer) once, after which they keep writing to the old surface pointer without properly relocking the surface. This may have seemed to work fine back in the days the game was made, but on certain newer hardware/driver combinations it can lead to all sorts of graphical problems.
The option you mentioned above seems to force this unsafe use of surfaces even on prior Windows versions, which is why I'd recommend against turning it on. If you turned it on by accident and have trouble turning it off (due to graphics problems), you can still turn it off in nox.cfg: set "UnlockSurface" to 0.