You cannot do this without having the Mac OS X installer from which to boot. Unless you made a copy of the installation disks to another drive, you will need the installation disks. You really should provide them with the sale in any case. If your Mac shipped with Mac OS X 10.6.x, you can call Apple support and ask to purchase a new set of the install disks. If your Mac came with an earlier version of Mac OS X, you can order a retail copy of 10.6 from the Apple Store.