Do you have reason to think it is correct and is full, i.e., was it slowing down and giving spinning beachballs? Or do you know the HDD was not full and it is a mistake?
You can start from your system disk (unless you have installed Lion and then you start from Recovery Disk). Then you can get at the disk and delete stuff to make room - http://www.thexlab.com/faqs/freeingspace.html