Everytime I do an update to OS X, I have to run these commands in Terminal, because the animated alarm clock in iCal is just obnoxious. (It also pulls precious CPU cycles.)
Taken from this hint at macosxhints.com, these are instructions for 10.4 Tiger:
cd /Applications/iCal.app/Contents/Resources/iCal Helper.app/Contents/Resources/sudo cp -p alarmclock.mov alarmclock-mov.BACsudo echo "" >alarmclock.movcd /Applications/iCal.app/Contents/Resourcessudo cp -p alarmclock.mov alarmclock-mov.BACsudo echo "" >alarmclock.mov
UPDATE: I posted instructions for 10.5 here at macosxhints.com.