Mac Locks up

I have a Quick Silver 1.25 CPU with 1.5 GB of ram when i go to certain websites that contain a video ( Ex: Trying to watch a movie trailer/preview )
My QS 1.25 will lock up, causing my mouse to not be able to close any programs or shut down the system and i eventually have to reboot.
I believe my system has a 64 GB video card if that makes a difference. Help ????!!!

When was the last time you updated the browser and flash player and then repair permissions.