If you have already installed the windows 7, make it boot on the dvd of windows 7 , go to customized options and there will be a menu for where do you want it to install (with some options to format and delete partitions)
1)Put the windows dvd
2)When it ask between Upgrade and Custom choose custom
3)Format the partition where windows is
4)Follow the rest of the BS
5)Change Mac and etc
P.S: There's some tutorials how to do it on the net