just got new imac with fresh Mavericks install, old data still remaining, how to find and delete? I just bought a 24' used iMac. 

The previous owner reinstalled Mavericks and sent it to me.

I looked at the disk usage info and it appears as if all the old data, movies, music, and everything else, is still there. Only 100gigs are free on a 500gig drive.

Is there a way to find this old data and reclaim my hard drive space?

I don't have a disk.
You do not need a disk to reinstall Mavericks, since it has a recovery partition that will download Mavericks and do a fresh install, if you prefer to do that.

Erase and reinstall Mavericks: http://support.apple.com/kb/PH14243

Otherwise, you can probably just delete the user account, which will offer to delete all the user's info, freeing up considerable space.