did your friend include the OS installation disks? Were installation disks for the software also included?
if you don't want to wipe out whatever software your friend passed along, then you need to keep the installation intact. Go to versiontracker.com and get "ChangeShortName" which will allow you to change the admin login name. Do an "archive and install" and delete the previous System, after verifying that the crucial software still works. You may have to re-enter some serial numbers, so be sure you have those.
If you don't want to do the archive-and-install, establish a second Admin account with a name that you like. Login as that user from then on. Yes, you'll have 2 admin accounts on the laptop, but that's no big deal, just make the passwords hard to guess. The original admin user will be moth-balled--above all, DON'T DELETE the original admin account. All sorts of problems if you do.