How can I stop Windows 7 64 bit from crashing every few hours?

For some time now I’ve been using WIn 7 64 bit version.
In the beginning it used to freeze after every 5-6hours but recently this happnes after around 2 hours of use.

The screen freezes whilst sound keeps playing. No BSOD.

I called Dell customer service and they replaced the motherboard and graphic card bu the problem still persists.

What can I do?