Run the following from a terminal in Ubuntu, assuming you have sudo privileges and "/dev/sda" is the device file for your hard drive.
sudo fdisk -l /dev/sda
Here's an example from my system.
Device Boot Start End Blocks Id System
/dev/sda1 1 19440 156150000+ 7 HPFS/NTFS
/dev/sda2 19440 19683 1953125+ 82 Linux swap / Solaris
/dev/sda3 19683 19696 99968+ 83 Linux
/dev/sda4 19696 38914 154368098 5 Extended
/dev/sda5 19696 22127 19531219 83 Linux
/dev/sda6 22127 38914 134836816 83 Linux
Here, "/dev/sda1" is marked "HPFS/NTFS". This is my Windows partition. If your Windows partition did not get deleted or formatted into a different file system, you'll also see one like this.
Typically, new computers come with one large windows partition that takes up the whole drive, optionally with a rescue partition. Hopefully you have a Windows 7 CD/DVD in case you need to recover the Windows MBR. Anyway, in order to install a new OS on such a system, you have to make room - either by deleting/formatting the existing Windows partition or resizing it. Hopefully you resized your Windows partition to make room for Ubuntu.
If your Windows partition survived, the problem becomes one of reconfiguring grub. Hopefully the following documents are helpful.
Arch Linux Wiki - GRUB - Dual booting with Windows
Here's my GRUB entry for Windows XP that Ubuntu added automatically
# This entry is automatically added by the Debian installer
# for a non-linux OS on /dev/sda1
title Microsoft Windows XP Professional
Leave a comment on results. I'll edit this answer with more details if you have trouble.