load an Ubuntu live cd, when it boots up, plug your phone in.
turn on usb storage in your phone.
run gparted (I don't remember if it is in the menu, but you can just open a terminal and type gparted)
change the bar in the top right from hda to the other one (either hda to sda, or sda to sdb)
it will show how your sd card is partitioned. if it is reporting the same size as your sd card, you know you are at the right place. right click on your existing partition, and delete it (note: this will delete everything on the sd card, you may wish to back it up first)
then create 2 partitions, one as fat32, one as ext3.
And you are done. As a bonus, you have Ubuntu running on your computer and dont have to use windows anymore