Assuming you've copied over all data you want to keep already.
Apart from The weatherman
's suggestion, if you want to leave the OS intact, you could reinstall the OS by booting from the install disk and repartitioning/formatting the drive before the install. You would also need to reinstall any software and drivers if you chose to include those as well.
If you just want to wipe the drive and leave nothing on it, you could use GParted to destroy and recreate the drive partition(s). You can find the link to it here:http://www.bleepingcomputer.com/forums/topic3616.html
(under Hard Drive & System Utilites
Also the hard drive manufacturer may provide a wiping utility on their website. This would "zero-out" all the bits individually and would be the most secure method, but also the most time consuming. Note that drives have a limited lifespan, and if you put older drives under extensive/high-stress operations, you might damage it ( as I did with my wife's laptop
); in which case the previous post would ultimately have been the best choice.