I'm using Ubuntu 12.04 unity (the problem was there before i upgrade) with an Intel® Pentium(R) 4 CPU 2.66GHz and my problem is that the CPU usage gets to 100% whenever i open an application and sometimes it crashes this is a snapshot of the system monitor : http://farm8.staticflickr.com/7078/7042247815_480c37aeb0_b.jpg
if there is a way to lower it i ll be thankful if you tell me
You could try unity 2d a go to see if that helps.
Log out of your current user and click on the Ubuntu logo next to your name on the login screen, next choose Unity 2d and then type your password and log in.
If this does not solve your issue you could try installing "Gnome" from the Ubuntu software Center and log into "Gnome Classic" using the same method as above (Although selecting Gnome Classic instead of Unity 2d)
Hope this helps.