You don't need to reinstall. When your computer starts, the first thing it does it look for something to boot, in your case, it's your internal hard drive. Then it looks for the master boot record on the drive (where the bootloader is stored).
What is happening now is that ubuntu replaced your windows bootloader with grub2. That's all (no other damage). And all you need to do is replace it.
In order to do this, you have to boot off some other device. In your case, the CD ROM. But you need to tell the computer to do this.
Two ways - most modern computers have a Boot Options key (it's F12 on my computer). When you first turn it on a screen briefly flashes and shows two things - a key to enter BIOS and a key for boot options. If you can get the boot options key, then it will give you a menu, and you select to boot off the CD/DVD. That's the easiest.
Once you're booting from your Ubuntu CD, select "Try without installing". Then once it loads the desktop, Click on Applicatios, Accessories, Terminal. This gets you to the command line.
You'll see a popup with some info, tab to OK and hit enter. You can ignore the info - that refers to a different use of Lilo than what you'll be using.
sudo apt-get install lilo
Then install the lilo bootloader in a form that is equivalent to Windows.
That should be it, "exit" from terminal, shutdown, removing CD, and reboot.
sudo lilo -M /dev/sda mbr
If you want more explanation, just ask.