I installed Ubuntu from a usb, but when I rebooted my computer after the installer finished, my computer booted into Windows XP. I didn't see any menu for selecting the operating system during boot.
How do I boot into Ubuntu?
As soon as you see the BIOS screen hold
If this doesn't work then you might have to repair GRUB by booting into a live CD.
Then open a terminal and type:
Mount the partition:
Now install GRUB in that partition: