how to get maps into satnav?

just bought a 7" sat nav but didn't come with any maps software thou it did come with an sd card and lots of software on it, but no maps.
it runs the win ce 5.0 system so will run on most mapping software.
i have bought some map software but what worries me is if i try and put it onto the sd card that come with the sat nav it may write over the software already on the sd card.
ANYBODY PLEASE HELP. THANK YOU.