I know you can change the rez of full screen, but i don't think you can change it into a window if that is what your asking to do.
Changing the rez is done by adding a coupe lines into the setup config of your game, under [GAMERUN] just add the lines
width=800
height=600
or you could add
width=640
height=480
im not sure but adding depth=16 might reduce the color also (i haven't tried this one yet)
doing this would change your fullscreen rez on game start up and you would still have to capture video in fullscreen mode and lowering the rez will produce a dirtier looking image on screen but could give you the extra processing power you need for your recording program to get a clip at a reasonable framerate.
