or if you don't want to be caught out at some point by it finding an old saved game from the previous owner on the hard drive you could just format the drive
1. Start your Xbox 360 console without a disc in the disc tray.
2. In the System section of the Xbox 360 Dashboard, select Memory to view the Storage Devices area.
3. In the Storage Devices area, select Hard Drive if you want to format the hard drive or other storage device, and then press the Y button on your Xbox 360 controller or on the remote control.
4. In the Device Options area, select Format. You receive the following message:
This will delete all content on this device. Do you want to continue?
5. To format your Xbox 360 Hard Drive, select Yes.