I just installed Ubuntu 11.10 on a friends Dell Inspiron laptop. I normally boots up to Windows Vista Home Edition. As part of the install procedure I reduced the size of the largest ntfs partition to allow space for an ext4 root partition and a small swap partition. After the install completed it prompted to restart and what followed was a reboot with Windows coming back up without any multiboot menu screen. Where is the grub boot menu?
You'll need to install GRUB manually. To do that you'll have to boot up using a live CD then do the following:
Setup a chroot environment.
open a terminal and mount the root partition to /mnt
sudo mount /dev/sdXy /mnt
sdXy being the root partition of your installed ubuntu, e.g. /dev/sda5. you can find that out using
sudo fdisk -l
next, mount some virtual devices
sudo mount -bind /dev /mnt/dev sudo mount -bind /proc /mnt/proc sudo mount -bind /sys /mnt/sys
chroot into the installed system
sudo chroot /mnt
finally, (re)install GRUB
grub-install --recheck /dev/sdX
sdX is the internal hard drive, probably /dev/sda
and while we're at it, we may as well update it, too.
if there were no errors during the installation, you can type "exit" to leave chroot nd then simply reboot the computer. you should then be able to boot both windows and linux.