is it possible to install another version of Ubuntu in my newest version 11.10? Something doesn't work well in this new version; see my post. I may need to switch back. Thanks a lot!
I don't know about install "in" unless you use a virtual machine. You can install "over" by getting a 11.04 CD and installing like you did 11.10.
You don't happen to have a separate /home partition do you?
Either way it seems like backing up your data and doing a fresh install is what you're looking for.
If you're fortunate enough to have a separate /home partition, when you install select the "manual" or "something else" (no really, that's what it's called in 11.10). Then Highlight the partition you want as root (/), set the mount point to /. Then select the partition you want to use for /home, and set it as /home.
This will let you keep your personal data, while installing the new OS.
If you don't have a separate /home partition do a standard install and consider making one with gparted.