you'll need to resize your windows partition, which is no simple task. you should be able to use fips or i had better luck using 'partition resizer' both of which are DOS programs. Defrag windows first and all that.
You'll then need to boot to linux and use diskdrake i guess (or cfdisk etc...) to create a new linux partition in the space you've freed up. This is where i'd start to get seriously confused, as i presume your new partition will be before the new one, and as such the /dev/hdaX number will change for the existing partition on the next boot, so you'll need to manually edit /etc/fstab to make sure it loads the right partitions, and also, to reconfigure grub or lilo or whatever to make it booto to the right partition.
Once (IF?) this section is done you should then (as a suggestion) move most of your system onto the new partition and maybe just keep the old partition as /home for example. (defined as the mount point in /etc/fstab).
ideally you'd be best just having the /usr part on i's own partition, but it shouldn't matter too much.
hope this gets you started...