Fair warning: You're about to partition your Mac's hard drive. The process of partitioning a hard drive with Boot Camp Assistant is designed not to cause any data loss, but when computers are involved, all bets are off. The partitioning process alters the way data is stored on your drive. If something should unexpectedly go wrong during the process (such as your dog tripping over the power cord and unplugging your Mac), you could lose data. In all seriousness, plan for the worst, and back up your data before doing anything else.
I mean it. Back up your data. I'll wait. If you haven't already, try using Time Machine to back up your data. Time Machine is included with Mac OS X 10.5 and later, and it's very easy to use. You can also use the third-party backup software of your choice. The important thing is to back up your data on a regular basis, including now; how you do it is up to you.