I think your file system on the drive are not set correctly for Ubuntu becuase of how Windows 7 sets up the partitions and drives. Take a look at these two very good articles on how to set up Windows 7 with Ubuntu:
lifehacker.com/5403100/dual+boot-windows...u-in-perfect-harmony
http://ubuntuforums.org/showthread.php?t=1035999
If you have done your dual boot this way and you are still having problems, let me know. I would try the lifehacker link way to do the dual partition as it lets you share documents between the Windows and Ubuntu.