One more thing to try...I just noticed the video card in your signature. Is this the system you are running? In that case, the problem may be the drivers for the video card.
Somehow you need to get to a terminal to enter the commands manually. When you get to GRUB, is there an option to boot in recovery or low graphics mode? Try that. THen select your terminal program or press CTRL-ALT-T to bring it up.
If that doesn't work, continue to boot to your purple screen and press CTRL-ALT-T and see if that brings up your terminal.
If you can get to terminal using either method, enter the commands at this link to install Nvidia drivers.
Here is a link that tells you the commands:
http://www.techlw.com/2012/06/install-nvidia-driver-in-ubuntu.html
Once you have completed this, try reboot and see if this works. If it is still not working, try erasing the nomodeset command.
One last option would be to try reinstalling using a different method, such as a USB stick.