|Projector only has s-video and composite? How old is it?|
Because you're displaying content on a large screen, you want the best picture possible. Out of the two connectors you've mentioned, S-Video will produce the best image.
Since you're using a low-resolution, interlaced analog connection, you'll have to drop Windows' desktop resolution to 640x480. Set the refresh rate to 60Hz, if it isn't already.
Pentium M oc'ed to 2.82GHz, 667FSB, 2MB L2
3GB of DDR-800 @ 667MHz
Modified PowerMac G4 'Quicksilver' case
HDMI to homebuilt 1080p projector