Assuming you don't want anyone recovering the data?
I don't think there's a fool proof way to completely erase all data without making it nonrecoverable, the best thing to do would be to format the drive a few times, and then write zeros to the disk.
Search the forum man. I think there have been a lot of discussion about this already with useful links and such. Essentially, write 0's (or 1s or a or b...) to the disk many many times over and over again..
I'm back, and its not that I don't want people to see the data, its that I cannot reinstall XP correctly, because I can see all the old data, but I cant access it...