"Extended partition," which it said is not empty, is one that contains one or more logical partitions. So it sounds like you may have created an extended partition and then a logical partition within it.
Is it possible to post a link to an image of what Disk Management shows in the lower, graphical section for this drive? If not, can you look for a logical partition inside an extended partition, delete the logical, and then delete the extended?
WARNING ONE: Back up an image of your Win7 first, so that if any of this breaks Win7 you can restore it.
WARNING TWO: There may be a Ubuntu boot manager now. The symptom would be if you finally delete that pesky partition and now the OS can't find anything to boot. If that happens, first boot off the Win7 DVD and try a repair installation and, if that fails, run around in circles screaming.
Edit: I am not a specialist. On the other hand, I have never rendered my PC unbootable. This is because I store image backups of my system drive. If I screw anything up, I pop in the backup volume, boot to DOS, restore the image, and I am no worse off than when I started. I highly recommend this approach to everyone.