If you are unworried about the family member being able to access your data, all you need to do is reinstall Windows XP from the original disk...a guided process that takes about an hour. However, if you want to securely erase your data, the process becomes a lot more tedious.
Reinstalling the OS simply hides the data...an experienced compuuter user, or just someone with a good program, could still recover most/all of your data. To completely wipe the hard drive, you should run a progam (such as WipeDrive) that completely erases your data, replacing it with a random combination of 1s and 0s. It is also recommended that you run the program at least 3 times (the government does it 7 times) to make sure data such as passwords, bank info, etc. are completely erased.
This process can take a couple of hours (though pretty much automated), but is well worth it if you want complete privacy/security. However, it is sometimes cheaper/easier to simply smash the hard drive with a hammer a few times and buy a new one. It just depends how private the data was, and how much time/money/effort you are willing to put into it.
Hope this helps,
I have a computer now that I'm giving to someone else in my family, but before I give it up, I want to clean it up, I just don't know if I'm ready for the struggles of a reformat. Is it really THAT tough? It's running Windows XP. Thanks.