Is there a way to squeeze more hashing rate from a weak hardware? I have a computer has two graphic cards ATI Radeon HD 5450, which are almost not used at all. 

The computer is running permanently and is used only for office work and data storage.

I tried to setup the minig there and I was i bit disappointed by the hashing power.

GuiMiner was hasning at 2 x 10Mh/s, CGMiner is a bit better, it does 2 x 14Mh/s. 

Are those graphic cards so weak that it is not possible to squeeze a bit more out of that?
I can get a third of this hashing rate on the CPU of my 5 years old notebook.

I use just the default settings and I mine by executing cgminer -o host:port -u user -p password -l 1 -T
12-14Mh/s is about right for the 5450. It's not that the card is weak, it's just that you're trying to make it do something it was never designed to do.