You can run a windows/linux system. And yes, you need two partitions, actually three is better since it's nice to setup the linux swapfile in it's own partition. The current installs of linux are nice, and have come a long way in terms of ease of install. Most can boot off the cd and walk you thru the install. But I would expect to spend a few hours reading the HOWTO's before you do your first install, unless your doing it on a spare computer, then, what the hell, go for it.
It should also setup boot software that allows you to pick the OS you want to boot. The linux booter is ok, but when I set it up I used Power Quest's partition magic, with boot magic, both programs make setting up the partitions and dual booting a breeze. It can resize your windows partition, and create linux partitions and the linux swapfile partition, all without loosing any data. All you have to do is free up disk space, but I would recommend defraging after that. I highly recommend partition magic (I use version 4.0, v5.0 is out but I can't imagine the new features are worth any extra cost as 4.0 has all the functions you would need).