I had issues updating to 13.04 from 12.04, so I went for a clean install of 13.04. the installation process went smoothly, and Ubuntu prompted me to restart in order to complete the installation. When I clicked on restart, instead of restarting, I got a black screen. after about 10 minutes, I shut it down, and booted up. When I was using 12.04, I could select Vista or Ubuntu. now, it automatically boots windows.
You problem is that the GRUB entry in your MBR is no longer working. To work around this, you must use