Urgent Help with after effects! please help me solve this problem A.S.A.P

Well.. after effects cs5.5 loads up perfectly fine.. if i dont do any editing at all it is fine. But once i start editing it's goes all fine for about 30 seconds.. for example if i want to use the green brush

tool.. it works etc but then suddenly it freezes. when i try and use the tool it wont do anything! the hole thing freezes then i have to do ctrl + alt + delete.. It keeps saying not responding.

I need urgent help.. i don't know why it keeps doing it! it does it when I use anything but it just freezes.. my pc is a gaming pc pretty good.. dual core etc 64bit! also dont say that my pc isnt good enough because it is! i could use cs4 on my laptop and that was alot lower specifications.. ive updated my nvidia drivers etc..

Someone please help, I miss my editing .