I recently updated my OS from Win7 to Win10. I can't play BG2 anymore. It's lagging ingame, the screen is flickering slightly and certain desktop symbols show through. Do I have to install the game again with the new client or is there a patch available?

The game works fine in windowed mode. I just don't like windowed mode, especially because it's fixed to 800x600 (or so) and I'd have to install a mod to fix that as well.

Before we go any further, it is important to clarify which version of the game/s you are talking about in forums which deal with multiple games and versions. In particular, state if it's vanilla, TuTu/BGT or enhanced edition. Also state if you are running the game through Galaxy or not.

The Enhanced Edition does not have issues like this and Tutu/BGT is all the Baldur's Gate II engine.

OP: There should be a configuration utility (either there's a shortcut or you find it in the game's directory) run it, look for 3D Acceleration and enable that.

Irrelevant to my point.