Try this:
1) Make a back up of your important files (Just to be on the safe side)
2) Run Disk Utility (You may have to boot to your OS X DVD and run Disk Utility there, I can't quite recall)
3) Click on your HD in the list on the side.
4) Click the "Partition" tab.
5) Select the Windows partition in the Volume Scheme Diagram. (MAKE SURE you have selected the Windows partition and not your Mac partition.)
6) Click the minus button below the Volume Scheme Diagram.
7) If it doesn't resize automatically, click the bottom of your Mac partition in the Volume Scheme Diagram and drag it down so it fills up the Volume Scheme Diagram and shows you ONLY "Macintosh HD" (Or whatever you have it named as).
Let me know how/if that works out for you. Again, you should be able to do this without booting into your OS X DVD, but it's been awhile since I've done it, so I could be wrong.