Desktop Computer Boot-Up Problem

I need a Tech that can walk me through a computer boot-up problem. I am on msm and yahoo instant messenger to communicate to get computer back up running. My other Desktop computer cannot boot-up because the the C drive is about maxed out. It is 80 megs and filled. Each time I try to boot it up screen says: UNMOUNTABLE_BOOT_VOLUME I have tried safe mode to no avail. I think it is because c drive is Full. At bottom it says: Technical Information: *** STOP: 0X000000ED (OXC0000006, OX00000000, OX00000000)