Windows Vista frequent Not Responding

Hi,
My Windows Vista frequently not responding "frezz" . If I do one thing, I
have to wait until it's done. If I click a second time on any open program, I
get that "Not Responding" error message and I have to wait about 10 to 15
seconds or more . This is very frequent.
my system events dont register any thing and i cant konw what is the problem

my pc is quad 2,2 GHz with 5 GB of ram and with gforce 500 mb of ram

i check my hardware and every thing is working fine

Thanks for your help.