Glad to see another person making the switch over to Linux! As per your dilemma, if you're wanting to keep an existing Windows installation and make space for Linux, you're probably going to need third-party partitioning software. There are several ways to deal with the problem though:
1. Buy a second hard drive to install Linux on and keep the OSs separated from each other.
2. Backup your Windows data and reformat/reinstall Windows from scratch (probably a good idea anyway, seeing how Windows will grow "stale" after a while). Plan out what partitions you want to make before installing anything, then just make your Windows partition. It's a good idea to install Windows before Linux (Windows will overwrite the MBR and your bootloader if you do it in reverse order). After you have Windows installed, install Linux on the remaining space - most distributions come with partitioning software that loads during installation to create partitions and to format them..
3. Some Linux distributions come with a utility that will resize your existing Window partition during installation to make room for Linux, but I've personally never tried doing it in this manner and have read warnings against trying this (if you want to take the risk - backup your data first!).
4. See if you could borrow a third party partition program from a friend - you're only going to be using it one time then uninstalling it, so I don't see the harm. I think I have one burnt to disc - if you need it, PM me.
I strongly recommend using 1 or 2 to resolve your problem (I used #2 the last time I reloaded my PC). Out of curiousity - what Linux distro(s) are you looking to install?
Edit: Might want to check out following thread, as it contains information on a simliar situation:
http://www.pinkpt.com/forum/viewtopic.php?t=14190