ubuntu 11.10 is a bullshit

Downgrading is not supported, so you'll have to do a fresh installation. If you have another operating system installed alongside Ubuntu, or a storage partition separate from the Ubuntu system, then you should remove your Ubuntu partitions before performing the fresh installation. Otherwise, after you offload any important files, you can just boot the Ubuntu CD and tell it to install using the entire disk.

See https://answers.launchpad.net/ubuntu/+source/ubiquity/+question/176778 and https://answers.launchpad.net/ubuntu/+source/ubiquity/+question/176816 for related discussion, if you're interested.

By the way, if you just don't like Unity, you might consider using a different interface. There is GNOME Classic (GNOME 3's legacy-like interface, called "GNOME Fallback" in many other OSes), which looks like (but is not the same as) Ubuntu Classic from Ubuntu 11.04 and earlier. Xfce also resembles Ubuntu Classic and GNOME Classic, and LXDE somewhat resembles it (and can be configured to resemble it further). KDE is not just a new interface, but a separate desktop environment that provides different applications, but you can generally run GNOME apps in KDE if you have GNOME installed too (which you do). And there is the GNOME 3 Shell, which is sort of like Unity, which does not look any more like a GNOME 2 desktop than does Unity, but which you might prefer.

For GNOME Classic, install the package called gnome-session-fallback and select "GNOME Classic" or "GNOME Classic (no effects)" as your session type at the login screen (to bring up the drop-down menu for this, click the gear icon). For Xfce, install the package xubuntu-desktop and select Xubuntu (not Xfce) at the login screen. For LXDE, install the package lubuntu-desktop and select Lubuntu (not LXDE) at the login screen. For KDE, install the package kubuntu-desktop and select Kubuntu (not KDE) at the login screen. For the GNOME 3 Shell, install the package gnome-shell and select GNOME at the login screen.