stop mouse going to second monitor when in game

[unwashed potato!]

for some reason when i play any game it doesn't lock the mouse to the monitor and the mouse can move from the game screen to the desktop in the other screen and if clicked this freezes the game.

at first i would just disable the second screen but getting annoying have ti disable and enable it constantly so figuring there must be a better way.

any one know how to stop this?