Whenever I render through the viewport, it renders perfectly, but when I want to render the image to .PNG for example, it takes longer to build BVH then it crashes. How do I change this. Any help will be greatly appreciated.
impossible to tell from this. your going to have to post/upload/link your .blend so that folk can take a look at your settings.
Sounds like a memory issue. What version of windows is it? 32bit or 64bit, what gfx card?
If windows 7 32 bit try running the following command as administrator and reboot computer.
bcdedit /set increaseuserva 3072