I got to the same point and found a way through, hope it works for you as well.
My hardware is a Gateway GT2222m with a quad proccesor and lots of RAM. Mainly using Windows I downloaded the Wubi thing to be able to switch between Win and Ubuntu 12.10 months ago.
Then came the update notice prompting to update to 13.04 I accepted and it seemed fine until it hanged somewhere while updating "ubuntu docs".
After that it got to the purple screen and the logo and it hanged.
So this is how i fixed it:
After reboot you are prompted to choose among windows and Ubuntu -if you have a similar install to mine- You chose Ubuntu and following comes a "GNU Grub version..." menu thing with two choices. The second one is an "Advanced options..." etc; I choose that.
Then comes another menu with several lines like: "Ubuntu 13.04.xxxxx...". I choose the one top most mentioning "recovery mode" in its name.
Then I came to a maintanaince Menu and chose an option named: "dpkg recover broken files". Then came a confimation box with some warnings, I accepted (like I had a choice..) an it went working.
I could see it was rebuilding every file in the install and it stopped twice to ask me if i wanted to download some of them (mainly the infamous "ubuntu docs"), once it came back and told me files were not there to download so I would have to do something else later (of course no details...). I confirmed everything to go on until it was done.
Took about 20 minutes but it was preety automatic.
Then another warning box and I came to a terminal like mode where i should log in typing my user and password.
Left with a command line I typed: sudo reboot
Typed my password and the PC rebooted to normallity updated to Ubuntu 13.04
Hope it works for you.