I dual-booted with Windows a while ago. Now I want to delete Windows (already did it) and make elementary take the rest of the hd without having to reinstall the system.
I think it should not be a problem.
here's one link how to live resize main partition
I'm sure there are easier ways to do it with gparted or similar software (haven't done it myself in a while)
Or you can just mount it as a separate partition and use it for data storage.