free up space to reinstall OS X through installation CD/ can't login

Hi little help.. i dropped my Macbook Pro and turned it on and it now prompts me for a password when i never used to have password prompt upon turning on. my usual name and password don't work, and even when i put in installation CD and change password it still won't let me login with the new password. so first question is could it be my name is wrong to enter along with the password ?? any way to change my name on it

after all this i decided to reinstall os x and do archive and install to back up files. problem is the installation requires 13 gigs free and even if i delete all apps still not enough to install as it won't let me delete other files to free up space just apps . is there anyway to delete files to clear up space to reinstall while still not getting access to mac such as disk utility?

G;day and welcome to the forums.

Suggest you take your MBP to an Apple dealer and have it checked out. Not normal behaviour at all and the hard drive may have been damaged in the fall. The drive is way too small anyway from your post and you should always have a minimum of 15% free space for the OS to work anywhere near its best.