Are you booting to the windows 7 install disk and selecting Custom Install, Drive Options to allow windows to format the new drive? Read this for instructions--
I have a new HDD and Mobo,I have tested all the cpu,hdd,etc, all is well. When I try to install clean do not have any options. There is not a custom option choice!
With a new (non-Dell) motherboard its not a Dell system anymore.... Check if your motherboard manufacturer even supports Windows 7.