Windows 7 was pre-installed on my machine. Ubuntu 11.10 doesn't boot after I installed it. Once Ubuntu finished its installation, I restarted.
After that, it just boots straight to Windows 7, no sign on Ubuntu at all.
If there was no error while installing then you can re-install your GRUB as follows:
Something likely failed with the Grub installation.
There are multiple ways to tackle this.. but ill give you the short one
DRIVE is the drive first starts as selected in the BIOS.. usually this is /dev/sda