Welcome to Ask Ubuntu! This is a rather common question, people install Ubuntu alongside Windows, but Windows still boots without offering the option of Ubuntu. This is often caused by GRUB/GRUB2 (the bootloader that allows you to choose which OS to load) not being installed in the Master Boot Record (the place that tells the BIOS what to boot).
The fix usually is pretty simple, there are two relatively easy options (taken from https://help.ubuntu.com/community/Boot-Repair
1st option : get a CD including Boot-Repair
The easiest way to use Boot-Repair is to burn one of the following disks and boot on it.
Boot-Repair-Disk is a CD starting Boot-Repair automatically.
Boot-Repair is also included in Linux-Secure-Remix.
Remark : you can also install the ISO on a live-USB (eg via UnetBootin or LiliUSB or Universal USB Installer).
2nd option : install Boot-Repair in Ubuntu
sudo add-apt-repository ppa:yannubuntu/boot-repair && sudo apt-get
sudo apt-get install -y boot-repair && (boot-repair &)
Then, once it's installed, do the following:
launch Boot-Repair from either :
(the Ubuntu logo at the top-left of the screen)
System->Administration->Boot-Repair menu (Ubuntu 10.04 only)
boot-repair in a terminal
Then click the "Recommended repair" button. When repair is finished, note the URL (paste.ubuntu.com/XXXXX) that appeared on a
paper, then reboot and check if you recovered access to your OSs.
If the repair did not succeed, indicate the URL to people who help you by email or forum.
Hope this helps!