If you mean that you installed Ubuntu or any Linux distro for that matter on the same partition that your Win 7 was installed on then
your Win 7 install is gone.
By that I mean the Ubuntu installer would have formatted (erased) your Win 7 partition and formatted it to ext3 or ext4.
The only recourse is a clean install of Win 7.
If in fact you installed it on another partition and Win 7 is still there on your original Win 7 partition then Ubuntu would have created a
boot menu with Ubuntu as the default entry.
It should have recognized your Win 7 install and created an entry for Win 7 as well.
You can reformat your hard drive during the Windows install process:
Reformatting your Hard Drive Using the Windows Installation Disk
Restart your PC and enter the BIOS setup by pressing the ‘Delete’ key. Once in the setup, ensure that the boot configuration is set to allow the CD/DVD drive to boot first.
Insert the Windows installation disk in the drive and then exit the setup after saving the changes.
As your machine restarts, you should be prompted to press any key to boot from the disk. It is important to boot from the disk because you cannot format your system partition from the operating system itself.
The Windows installation screen should take you through several steps before you are allowed to format an existing drive.
You can choose to reformat existing partitions, delete existing partitions, or create new partitions in unpartitioned space. Choose the required amount of space by specifying a number denoting the formatted area in megabytes.
You may choose NTFS or FAT32 for formatting; NTFS drives are faster, superior and more secure.
That's the easiest way for most people who are not familiar with the process.