I just purchased an ibook from someone and on the administrative account (only account) i cant change the short name. I changed the name to mine in system preferences, but short name is grayed out so i am unable to click in to to change it.
If you are not going to erase the disk and reinstall (strongly recommended for a used computer), just create a new administrator account with the desired name, logout from the old account, login to the new account, and delete the old account.
If the disk was erased, then Tiger installed, it is probably OK. Make sure you have all the disks that were provided with the computer when it was new, and the disks for any software that was added later.