Hard drive help plzz..?

I want to format one of my computers that has windows 2000 and instill windows xp. I dont know how to format the hard drive and i cant make a 2nd space in the hard drive. and when i put the xp instilltion disk it doesn't format the windows 2000. and i cant have a hard drive with 2 operning systems. is there a way where i can format the hard drive?