You must be storing the files on a Windows server with an NTFS file system. As of 10.6, a Mac will store files' resource forks as an NTFS alternate data stream rather than in an "Apple Double" file (the files beginning with "._"). You can force the OS to use the older behavior (my preference) by executing the following two lines in Terminal:
sudo echo "[default]" >> /etc/nsmb.conf
sudo echo "streams=no" >> /etc/nsmb.conf
You may have to unmount and remount the server, or possibly even restart. If you want to backup your nsmb.conf file first (which is probably empty or non-existent), execute this first:
sudo cp /etc/nsmb.conf /etc/nsmb.conf.old
Any files you copied with 10.6 before doing this should be copied back to the Mac first, then copied back to the server after making the change, to store those resource forks with Apple Double files instead of alternate data streams.