How do you manually determine Ct value? - (Apr/08/2009 )

Hi all,
I am new in real-time PCR and I have done some experiments with it, the software of the instrument calculate the Ct value automatically only after drawing an standard curve, what can I do to determine the Ct manually? Can I perform delta delta Ct relative quatification without a standard?

it would be helpful if you indicate in more detail what you are doing and which instrument and software you are using.