I am using AMD64 kubuntu with 2GB memory. Whenever i try to render video more than about 10 minutes length, the computer just shuts down without warning. I tried changing the tmp folder settings--doesnt work-- I want to know if it is a Kdenlive problem or a hardware problem. Please help.

Not enough information.
Which versions of kdenlive, mlt do you use?

You might have run into the memory leak, which was recently fixed (but not yet released).

http://www.kdenlive.org/mantis/view.php?id=1197