1. Uninstall any unnecessary programs, including duplicate antivirus programs, etc. which hog resources and processing power.. Disable unnecessary startup programs and services as well by accessing "services.msc" and "msconfig" and disabling those items which don't need to constantly run in the background.
You should be able to determine any culprits by disabling that particular program from running.. A common source of such is your antivirus/antimalware. Try disabling it temporarily.
2. You don't mention WHICH 64 bit operating system you're using but make sure you have the correct & updated drivers for the audio, video, and motherboard/chipset.
3. Tried changing the screen resolution? Do you have it set at the correct native resolution? If not, please do so.
4. If the problem is freezing of streaming Flash videos while on the internet, the solution below should help.. If we're still talking about offline viewing of DVD's and video files, then that's a different issue:
So first, check if you can see the Flash animation (banner) without the system crashing at:
Don't worry, it's not a video, it shouldn't crash -- if it does, this won't fix your issue. If it works without crashing, right-click on the Flash animation (banner) and uncheck "Enable hardware acceleration" from the Settings panel. You should now be able to view the YouTube videos without crashing the computer.
And on Windows 7 computers, some folks are finding relief for your issue in Internet Explorer, by opening the Control Panel-Internet Options-Advanced tab.. Once there, place a CHECK mark in the box next to "Use software rendering instead of GPU rendering", then click on "apply", then OK.
Hope this helps.