uninstall 11.10

how did you install Ubuntu Stoney? if it was from the Wubi windows installer you can use this guide
https://wiki.ubuntu.com/WubiGuide#How_do_I_uninstall_Wubi.3F

If you did not use Wubi and have a dual boot setup you can delete the Ubuntu Partition from within windows
http://www.makeuseof.com/tag/how-to-safely-uninstall-ubuntu-in-windows-dual-boot-environment/

Are you wanting to go back to an earlier version of Ubuntu? If so you can delete your Ubuntu Partition and reinstall the earlier version also.

TO delete a partition see
https://help.ubuntu.com/community/HowtoPartition
theres a link for Deleting partitions

Could you explain more previous configuration of computer ?

I installed Ubuntu using WUBI in October 2009, and having routinely been installing updates as they were made available. The result was a dual-boot system with Windows Vista and Ubuntu. I keep data files (which are accessible by both Windows and Ubuntu) on a separate disk drive. Backup is to a third drive.

In the process of downloading version 11-10, the download was interrupted several times, so the installation was corrupted, and is basically unusable.

What I hope to do is

a) uninstall 11-10
b) uninstall 11-4 if necessary. The interface is really much less convenient than 10-10.
c) install 10-10.

Does that mean that I need to delete the partition that Ubuntu is now operating from? Or can I just uninstall and then install 10-10?

to uninstall a Wubi install please follow this guide
https://wiki.ubuntu.com/WubiGuide#Uninstallation

you can then go here
http://releases.ubuntu.com/10.10/

and at the very bottom of that page you can download the wubi.exe installer for 10.10 for installation.

Support for 10.10 will end in April this year ( https://wiki.ubuntu.com/Releases ), therefore I don't recommend going back to it.

When you uninstall Wubi it removes the virtual disk which includes whatever release of Ubuntu you have. There are no partitions and you should avoid formatting or playing with partitions when you have Wubi. Follow the guide Nathan Heafner posted on uninstalling. This will completely remove any data that's not on that separate drive (programs installed and your custom setups including web bookmarks etc.)

If you have a separate partition you're using for Wubi, you could also install Ubuntu directly to it - it will make better use of the space and be faster (marginally) and more robust.