If others have Lion or a later version of OS X, you can erase and reinstall OS X from the recovery partition. See http://support.apple.com/kb/PH11273.
- Restart and hold command-R on startup.
Open Disk Utility and erase either the main OS X partition (like Macintosh HD, indented below the drive) or whole drive. If you have an HDD and not an SSD, you can erase the data securely by pressing the Security Options button and choosing for example the 3-pass erase option.
Quit Disk Utility and choose Install OS X.
It is not necessary to delete your user account before you erase the partition or drive.
If you don't have other partitions like a Bootcamp partition, you can just erase the main OS X partition. That's what the KB article linked above means by startup disk. If you erase the whole drive, it also erases the recovery partition which is hidden from Disk Utility. If you don't restart the Mac, the contents of the recovery partition will be loaded in memory and you can continue to reinstall OS X. If you do restart the Mac, the drive will now be empty, but if the Mac has firmware support for Internet Recovery mode, it can still start up in Internet Recovery mode, which means that it downloads a disk image for the recovery system from Apple's servers, and you can then reinstall OS X normally.
If you have an SSD, the Security Options button is grayed out. According to http://support.apple.com/kb/ht3680 erasing the drive normally might be secure enough:
Note: With OS X Lion and an SSD drive, Secure Erase and Erasing Free Space are not available in Disk Utility. These options are not needed for an SSD drive because a standard erase makes it difficult to recover data from an SSD. For more security, consider turning on FileVault 2 encryption when you start using the SSD drive.
I don't know if it's actually possible to recover data with software like DataRescue after erasing data from an SSD normally, but it might also depend on what SSD you have. To make sure that the data can't be recovered, turn on FileVault 2 from System Preferences before you erase the partition or drive.