partions

i'm installing debian 3.1roa
questions..

1. when installing how do i set it so my computer doesn't have ubuntu/windows as a partition anymore and so i can just have debian?..is it something i can do with the debian installer or do i need to do something else before hand?

2. can i install over another gnu/linux os? or would i need to delete ubuntu and then just keep windows and then install over the windows OS?