At least 10 GB of space on C drive, or on which Windows 7 is installed,
A CD or Bootable USB of Ubuntu.
If you had above things then follow below steps :-
Insert your Ubuntu CD or attach bootable USB to computer,
It will auto play, just run Wubi and then it will show you 3 option,
Just select the second one, Install Inside Windows, Then it will present a set up wizard in which you had to select the drive, in which your Windows 7 is install, then username and password, and allot at least 10 GB of space and then click next,
It will extract some files from CD or USB and once it finish it will ask you reboot your computer, first remove your CD or USB and then reboot your computer,
On reboot at boot loader you will get 3 entries, Windows Developer Preview, Windows 7 and Ubuntu, select Ubuntu to boot into it and continue installation,
Once you boot into Ubuntu it will install files and after that you will be asked to enter password to login, just enter it and you are done.