You could go and resize the partition, but that's iffy so do backups before using partition magic if you decide to go that route.
The most easiest would be to go and get a second-hand HD somewhere, maybe e-bay or something. A 6 gig'r would be plenty, if just a bit slow.
I guess that you have the windows HD plugged into the primary ide controller as the "master", then plug the linux in as the other IDE controller as a secondary master.
As long as your carefull you can keep either OS completely seperate and (if your bios supports it) use the boot device selection options to switch to either OS.
Then after you get familar with linux you can make the boot loader for linux have a option to boot the windows disk, to make stuff easier.
Then if you decide that you don't like linux you can format the 2nd hd and use it for the extra space. Hell, during the Linux install you can just unplug the windows drives completely (as long as you keep the linux HD/cdrom on the secondary controller).
Then no worrying about screwing up the windows boot loader, or frying the windows partitions, or filling up a windows partition with a linux sudo-filesystem file.
Just my 2 cents