Free up hard drive space?

hey my computers a few weeks old now and i've been tring everything possible to make sure there's no viruses etc and also trying to conserve space on my hard drive as it is a sharing computer and my last computer was wrecked from the amount of junk on it , does anyone know any good ways to get rid of the stuff you dont need or anyone know any files that stay on your computer or anything? i've done disc cleanup, wiped temporary files and cookies etc but the free space doesnt seem to go back up , just down. also i have windows mail (my gmail account) and i notice in the start up menu some of my messages are there like they dont just stay in your email but actually stay on your computer. would that be taking up space and how can i stop that from happening?