I hope there is a way to get everything off the harddrive - I have recently changed computers and have a lot of banking information and personal documents on the harddrive of the old computer. If I can't permanently erase data, that computer is not leaving my possession!
Sorry to threadjack a bit, but I thought I'd answer the data removal question. I used to work for state government IT. We had to remove lots of sensitive data from old computers before disposing of them. It is, in fact, possible to remove all data from a hard drive. Don't believe everything you see on CSI or other shows. They do things all the time which are completely impossible. (I now work in the criminal law field and some of the stuff they do with forensics makes my head hurt too.)
To confirm, just hitting delete does not remove any data from your hard drive. It removes the pointer to where the data is stored. You will overwrite this data if you add more data to the computer, but due to how hard drives are organized, you cannot guarantee that just saving more files will remove anything. So, just hitting delete is a bad option.
In truth, if you want to be 100% sure the data is gone and you're not an IT person, then pay someone to destroy the data. There are companies that offer this service. Find one that is bonded and has a good reputation with their customers and the Better Business Bureau. This won't be inexpensive.
However, there are three basic methods of removing data such that it cannot be recovered. Two of them fairly easily available to the individual computer user.
First is the physical destruction of the hard drive platters. (Do a Google image search for "hard drive platter" if you aren't sure what one looks like. They're the disk thingies.) Don't just take a hammer to the hard drive, unscrew the case and take the hammer to the platters until they break. If you have cutting tools you know how to safely use, that's even better. (There are machines that shred metal if you know someone who works in recycling.)
Second is the software solution. There are programs out there which will overwrite the data on a hard drive. (You may have to install the hard drive on another machine first, though some software will come with a boot disk so you can use it on the computer in question.) Basically, if you overwrite the data with all zeros, all ones, or a random combination, the data is gone. Some of the software will get really thorough and do all three of those one after the other.
Third is using a magnet. There are powerful electromagnets you can buy that are designed just the right size and power for hard drives. This will randomize the contents of the hard drive. However, this is an expensive device and probably not something you have lying around. This technique also has a theoretical flaw that you cannot guarantee you hit the entire hard drive with a hand-held magnet. I've seen recommendations that you use the software method first if you're using magnets.
If you want to be really thorough, you go the software route, then you use the magnet, then you destroy the platter. For your personal data, you generally aren't going to need to use more than the software method. Though feel free to use the physical destruction method if you never want to use the hard drive again and have some computer aggression to work out.
Brown University has a web page on data removal, including software suggestions. http://www.brown.edu/cis/policy/datarmv.php
Ending the Threadjack.
When I worked for the government, we had a guy who was just a little computer savvy and wanted to look at porn. So, he figured out there was an IP address not being used and would switch his IP to that in the morning and then back when he was done for the day. He was literally surfing porn for eight hours a day sometimes.
At least until IT got the usage report and saw that a server which had not even been installed yet was topping the charts for web usage. The network also recorded the hardware ID of the computer connecting, so it was about 60 seconds work to figure out whose computer this was.