|Just to elaborate, you choose Custom and then Partition/Format. Then you delete all existing partitions, create new one, format and install. I recommend if you plan on creating more than one partition, just set up one and do the other(s) later from within Windows (Disk Manager) since you cannot be sure if you have two or more, which one Windows will install on.|
On one build I did with Windows 7, I was sure I selected the correct partition, but it installed on the second larger partition that was supposed to be for files. I ended up shrinking that partition and creating a third, put the programs on the one originally for the OS, and the files on the third (C is still the system drive but it is physically the second partition). The three works and I could have started over, but I had just finished getting all of the updates, drivers, and preferences done and set up when I noticed the size of C drive and had to decide on what to do. It's been that way for over 2 years I believe.
You have to be a little bit crazy to keep you from going insane.