I wanted to OC my Celeron D with clockgen, but I'm don't understand how to use it. Could someone please tell me what I have to do to get it to overclock? Thanks.

google clockgen and follow instructions on their site.

I got it to work, but when I change the speed of anything, the screen flashes white and the system freezes.

that means you over overclocked and its unstable. try clocking lower than you tried.