this is a very very common problem.
there could be multiple causes, first is that your ubuntu username is not added in the samba group. that may sound like greek, but the fix is simple. just perform the following in the terminal:
replace ubuntu_username with your actual ubuntu user name
sudo smbpasswd -L -a ubuntu_username
sudo smbpasswd -L -e ubuntu_username
the other common cause for this problem occurs because of firewalls. if you've installed firestarter on your ubuntu machine, you will run into this problem unless you configure the ports correctly.