How do I disable full screen mode in all my GOG games? This feature is fine if I was only using one monitor, but I've got a dual monitor set up. Having the game in full screen mode makes it... difficult... to use my second monitor.

I don't mind digging into "the system" to change a configuration file. I use Windows 7...

For any DOSBox-based games it's as easy as changing a line or two in each of the games' config files. As for the rest, while some have a window option, most don't. There's definitely no all-inclusive registry hack you can use. I'm sure some might be doable if you poke around enough on a specific game's subforum or google, but be prepared to be disappointed. Many of those probably won't allow you to do anything like that. They're simply hard-coded like that, since most of these oldies were made during a time when a multi-monitor setup was uncommon if not unheard of.

Even modern games sometimes don't have a windowed mode.

For DOSBox, though, you don't even necessarily have to mess with the config files, you can just press alt-enter. Alt-enter toggles between fullscreen and windowed, though you might still need to adjust the settings if you want to change the window size.

You're wanting to permanently disable it? Here are the basics:

For DOSBox-based games, run the game's "graphic mode setup" tool in the Start Menu and uncheck the "Full Screen" option. Click "Save Settings" and you're done! Alt+Tab/Win+Tab will disconnect the mouse from the DOSBox window, or you can hit Ctrl+F10. If you don't want DOSBox to ever capture the mouse automatically you can change dosbox.conf's "autolock" parameter to specify this.

For ScummVM-based games, run the launcher and disable full screen in the settings there. I'm not sure whether ScummVM captures the mouse in windowed mode but the usual methods will get around this.

For native Windows games there's no single way to disable it. Refer to each game's individual forum for details. The sticky topics will often cover this sort of thing.

For Windows based games, there are two options:

1. The game provides windowed mode out of the box, usually (if it's not provided on the settings screen) by adding a command line option to it when it starts. There is no standard option that works for all games, but many games use "-window" or "-windowed".

2. DxWnd is a tool that intercepts a game's DirectX calls, and makes it run in a window while letting the game think that it runs in fullscreen. This is a rather hack-ish mode to force a game into a window, it's still a bit experimental, buggy, may require a lot of fiddling around with options, and doesn't work for all games, but it's often the only way that works. D3DWindower does the same, but I don't think it's still under active development, while DxWnd is.

Thanks everyone! These were the kind of answers I was looking for! :D