If you're running out of space on your NXT, open the NXT panel,, chose "Memory" in the software and feel free to delete the following system files.
RPGReader.sys
Demo.rxe
Try-Touch.rtm
Try-Light.rtm
Try-Sound.rtm
Try-Ultrasonic.rtm
Try-Motor.rtm
Woops.rso
faceopen.ric
faceclosed.ric
! Startup.rso
! Click.rso
! Attention.rso
You will lose, 1) the ability to use the "NXT Program" system, 2) the "Try Me" functions, 3) the funny sounds the NXT makes when it is turned on, downloaded to, or when you press the front panel buttons. To regain these files re-download the NXT firmware from the software.