I'm using Ubuntu 9.04 ("Jaunty"), so it might be a little different form Karmic, but there should be something in your System->Administration menu called "Log File Viewer". If it's not there, you might try looking in other menus.
If you can't find it at all, open a terminal (Applications->Accessories->Terminal, or just press Alt-F2 and type "gnome-terminal") and enter
cat /var/log/messages | grep "composite"
dmesg | grep "composite"
..and post those results (if any) here in [code] ... [/code] tags.
Hope this helps...