New to linux, after installing with Wubi I have screen flashes

Hello everyone in the Ubuntu community, I always wanted to have a linux OS in one of my computers. Finally I saw the opportunity to install it along with windows 7 using the Wubi. I succesfully installed Ubuntu, and get the logon screen, when I log in I just see the back screen and the cursor nothing else and the screen flashes.

I am using a laptop with a AMD Turion II ultra 2.5Ghz, 4 gig of ram, Ati mobility Radeon HD 4250, Windows 7 home premuim.

I downloaded the ubuntu-11.04-desktop-amd64.iso and used it with the wubi tool to install the ubuntu.