I had the same issue, running a MacBook Pro with a 100 gig drive, space is precious. For a straight-up answer to your question, you'll have to re-partition your drive with Bootcamp. But if you read on I'll explain how I install windows applications on my mac partition and run them without a hitch, allowing me to keep my windows partition fairly small.
Within Parallels, you can "share" folders, meaning you will be able to access your entire Macintosh partition while booted with Parallels. This even means that you can install new programs or games onto your "shared" folder which will reside on your Macintosh partition. I simply created a folder inside my Applications folder named "Windows" and install all my windows applications and programs in there. This means you only need to make your XP partition big enough for the Operating System, plus a little extra for updates, etc.
On the other hand, this will not work while booted in Bootcamp, as the Parallels shared folder is only active while booted with Parallels. Here's a great program
I use to get around this. Install this program while booted in Bootcamp and it will make your entire Macintosh partition visible, allowing you to install your games and programs on the Mac partition (I install it into the aforementioned "Windows" folder in my Macintosh Applications folder). Works great. Just run the windows setup program for the application you want to install, and when prompted, point the installer to the Mac partition instead of the default windows programs folder. I've installed 2 games this way and they run flawlessly. Just as though it were another Windows formatted drive in my computer.
Hope this helped! Feel free to ask more questions.