The link in Andy's answer has one answer but there are other ways to fix it.
When you install Ubuntu, it overwrites your Master Boot Record (MBR) with the program Grub that allows you to choose which operating system you want to boot into.
When you install Windows, it overwrites your MBR with the pointer to run Windows. This effectively means that when you install windows you can no longer access your other operating systems.
The simplest way to fix this is to always re-install Ubuntu after any Windows re-installation (which will replace grub as part of the installation process).
You may not want to do this as you may have files and configuration that you would lose.
You can also leave the Ubuntu partition as it is, and manually replace grub as described in Andy's link (reproduced here). However, in my opinion this is harder than just re-installing.
For future reference, it may be best if you seperated your root (/) and home (/home) folders into seperate partitions because then you can re-install ubuntu on the root partition without losing any settings or data on your home partition. This process is described in Partitioning your disks.
