iMac Freezes Often - Please Help

I have a 2Ghz Intel Core 2 Duo iMac running 10.6.8

I cannot tell you how many times I have been surfing on Chrome or Firefox and all of a sudden (especially when scrolling) the window will just freeze and my cursor will go lollipop. I can move my mouse around, but nothing is working... it's all frozen.

When I try to bring up the Force Quit window, it will not come up.

This has got to be a software problem, I just got this iMac and it is having the same problem as my older iMac (I migrated everything over).

Any help or way I can trace what is causing the problem will really help. Getting tired of force-quiting my iMac.