Update: The guide below is outdated. Follow this site for more info.
Users running Mac OS X with Bootcamp Windows may struggle to modify or update your documents and files in the Windows partition – usually it is in NTFS File System format which you can read the drive natively in Mac OS X but not write onto it.
Recently Amit Singh, a Google employee, releases a implementation called MacFUSE which makes it possible to use any FUSE (File-system in USErspace) file systems in Mac. And the most useful FUSE is the NTFS-3G Read/Write Driver, which ables system to load NTFS with read and write capability. This is truly the greatest news for dual booting Mac OS X and Windows XP or Vista.
Without going into great deal of technical details and compilation of the source code, I found out users around Internet already came up with binary version (in DMG) of MacFUSE and ntfs-3g, ready to install (credit to ShadowOfGed at AppleNova). Here are the instructions on how to use MacFUSE and NTFS-3G. It does require a little of administration skills as it involves running commands in the Terminal.
MacFUSE/NTFS-3G works for me, but as this is an experimental software, so back up your data, and try it at your own risk.