In a custom installation of windows 7, you can click 'advanced' under disk options.
you want to select the correct drive and delete all partitions on it. you will lose all data held in the partitions.
now create a new partition and describe the size in mb.
for example, if i have a 500gb hdd i want to split into 120gb and 380gb, and the drive is called 'disk 0', i would delete all current partitions ie disk 0 partition 0, disk 0 partition 1, etc.
then create a new partition sized 122880MB, and another sized ~389120MB (the remaining size will be less as the total space available will be <500GB).
you want to format the main partition and install windows there. once done, go to start->type "computer management"->disk management->select the ~380GB partition and format/assign drive letter.