what PC games are these that you're trying to play? Even the old first Wolfenstein 3D had mouse support. So did the King's Quests and the ones like it, at least the ones that didn't require you to type.
It's a question of what mechanics would be involved, and whether it would be possible to produce the necessary keyboard commands with a mouse. Would you be pressing up, down, left, right by moving the mouse in those directions? If so, would "up" stay pressed while the mouse was moving that direction, then release? In that case, how would you keep a sustained "up" pressed... you'd have to release it all the time when you had to pick the mouse up and put it back at the middle of the pad.
But, you could also have "up" be pressed when the mouse moved up a bit, then release it when it moved down... but then how would you press up without then pressing down when you tried to "release up"?
Then, you could also have the mouse hold down the keyboard key it'd been moved in the direction of, keeping track of a "center position" and the mouse's position relative to it... but if you did that, you'd lose track of the center position and never find it again. (I know all about it, I once tried playing Nintendo with the Power Glove. It's the exact same story as what you're trying to do.) Plus, you'd probably have issues related to the "mouse acceleration" feature in windows. If you moved the mouse quickly to the left and then slowly back to the center, you'd still be to the left of center - the center would keep moving around on the mouse pad. Same if the mouse ever skipped a beat, which they do, even if you turned mouse acceleration off.
Or, you could have the gesture system mentioned, where you move the mouse in a pattern and it sends a keystroke. This seems like the only method that would actually work for sending the key you want to send reliably, but it would be so darn slooow! You'd have to trace out a unique symbol on the mouse pad, just to push a single keyboard key. Add to that the fact that it's difficult for the PC to recognize gestures easily - look at the game Arx Fatalis, it's always reading your hand gestures wrong and doing the wrong spell rune. You'd never be able to play your game right.
There is one alternative left, and it was used to add mouse support to old Dos programs that used text-mode to provide an interface. For every certain number of mouse-ticks the mouse moves in a direction, it could send a single keypress command, i.e. press "left" once for every quarter inch you moved the mouse left, or something. That would work only with games that use the same type of commands to move the character, i.e. press left once and your guy moves one square to the left. I'm having trouble thinking of any games that worked like this. Almost all of the games I played had either had you press left to start moving and left again to stop, or had you hold down left to keep moving left and release it to stop.
So anyway, think about whether or not this is even possible before you spend a lot more time looking. What you should really do is use a joystick to do this, because it has an inherent natural center position that it returns to automatically, so it can use method number three without losing track of the center position, and works great.