Wubi is a free tool that allows you to install Ubuntu Linux on Windows as if it’s a normal Windows application.
Windows installation does not change. Wubi creates a boot menu and you can choose to boot into Windows or into Ubuntu without modifying the partitions or adding any special drivers. Wubi places most of the files in one folder. Therefore, if you decide not to use it anymore, you can uninstall it.
Download Wubi from http://wubi-installer.org/. Wubi works in Windows 98, 2000, XP, Vista and 7. To start installing Ubuntu in Windows, double-click the file you downloaded (wubi.exe).
For more details, please refer “Dual Boot Windows and Ubuntu”.