You need to use the partition editor, Gparted or similar, and first reduce
the size of home. Then from the freed up space you create a root partition
for 11.04. This should not take more than 20 GBs. You can use the same home
partition. You can either use a new user name and keep things separate or
re-use the one you have. I prefer a separate one just to be safe.

Then you install 11.04 to the partition that you created. Choose a manual,
custom or expert installation whatever they call it now (it keeps changing).
It will detect your existing installation of Ubuntu 10.10 and offer to
upgrade. Decline that option. You set the mount point for 11.04's root
partition as / on the created partition and use the existing home partition
as /home. Format ONLY the root partition. Check and double check before
committing to the changes. Use ext4 for both or whatever else you might be
using instead.

After installation you may want to set the default grub choice as 10.10
instead of 11.04 or even Windows. You do this with a text editor or use an
utility to do it.

I have booted at least 8 different OSes in the past. Things that can mess
you up are: some distros will not detect other distributions (Fedora and
Mandriva are bad for this, but Ubuntu is great). Windows of all stripes will
destroy grub if it is given the chance. An incomplete installation which
mangles grub is also a potential problem. Your installations will still be
there but you cna't enter them without fixing grub from the Live CD. You
make a mistake and install to the wrong partition (so pay close attention).
Or, you have some funky BIOS settings that mess up grub. You can fix this
temporarily be editing grub on the fly and then making it permanent once in.

It is not hard once you have done it a few times. You just have to keep your
wits about you and know which partitions are which and make sure that you
only format things you want to.