You didn't mention if you're using a dual boot, so I assume you only have Ubuntu currently installed to your system.
Option 1: If want a dual-boot...
..you'll have to install Windows 7 to a new partition. This install will overwrite the boot sector so that the previously installed Ubuntu will not be recognized during boot, and you'd have to fix this using Boot-Repair tools/disk.
Option 2: If you don't want Ubuntu anymore...
..you can delete Ubuntu partitions with Windows 7 installation disk and create a Windows' filesystem (
fat or preferably
ntfs) and install Windows there.
Option 3: Use virtualization software to install Windows OS as a guest in your Ubuntu sysem:
Among the ones available out there, I use Oracle's VirtualBox (wikipedea). This link explains how to install VirtualBox.