Use the Ubuntu installation DVD/CD as a 'Live' source of Ubuntu. Boot into Ubuntu from the Live CD (at worst you can download a copy of the Live Cd) and then select the Partitioning tool. Use this to repartition your hard disk to one partition and select it to format to NTFS. That should return you to your original hard disk configuration. Now turn off Ubuntu via the menu. Then reboot using your W7 DVD and it should install without any problems.
Your 'friend' could easily have installed Ubuntu as a dual boot with W7 and you could then have taken your pick of OS when you started at boot. Deleting W7 was a rather silly thing to do.