old game stuck in fullscreen and alt tab won't get me out

Win+D sometimes works when alt tab doesn't, it forces the desktop. Worth a try before the task manager route.

Try Alt+Enter first, lots of old games use it to flip to windowed. If that does nothing, Ctrl+Shift+Esc for Task Manager, and set Options > Always on top so it doesn't hide behind the game. For next time, check if the game takes a -windowed launch option, or run it through dgVoodoo2, which wraps old DirectX games and lets you force a window.