Wiping Hard Drive

Hi,

I have a Dell Optiplex GX100 desktop with 10g hard drive space. I want to free up some space on my hard disk. Are there some simple steps to wiping a hard drive? I have Windows XP. I also want to keep a lot of data that I have on my hard disk. In order to keep that data do I need to back it up before I wipe the drive?

Thank you in advance for your answer.