HI, try booting into safe graphics mode which you get to by choosing the second menu item in the Menu at Boot time. Once there, select DPKG but be sure you have a wired connection to the internet. Let DPKG do it's thing then shut down (NOT continue) and reboot to see if it helps. You may also want to change your 'Software Update' settings to point to 'Main Server' too so that you get all necessary updates. (Menu | System Tools | System Settings | Sofware Update - then after it has done it's whirring, click on the 'Settings' button and choose the first tab on the window that opens and change software source to 'Main Server'.
Keep us posted!