hippias - Welcome to LQ. It's unclear to me what the current condition of your 80G drive is -- is it entirely allocated to XP, with no available free space, or is it partitioned with half going to XP and the other half unallocated, free space? Obviously if it's the former, the installation will fail because there will be no room to install Linux into. If it's the latter, then you should be ready to go.
My recommendation would be to make the partitioning task separate from the installation. Believe me, it's much easier to deal with partitioning as a single project than it is to try to make it part of the installation. If you decide to go with 40G for XP and 40G for Linux, then at the end of the partitioning exercise you would want your 80G to be divided into 40G of NTFS, and 40G of unallocated free space. One excellent shareware utility program is BootIT NG
which can handle both resizing an existing partition, as well as other common partitioning tasks. Good luck with it. Once you've got the partitioning out of the way, the Linux installation should proceed smoothly. -- J.W.