I want to install Debian with LXDE on partition, that already have Ubuntu 11.04 installed. I have a lot of music there, and I don't want copy it to another partition. I know that Debian installer will propt that on this partition is already linux installed, but will it remove all from my /home/ folder if I type the same username in setup? Or should I have to encrypt this folder?
First, make sure you have backups. Accidents can happen.
The Debian installer will overwrite the partition that you select to install the system. Shrink the existing partition to make room for a new partition where you'll install Debian. In general, it's a good idea to have a partition for the system and a partition for your data.
Have you made your backup yet?
Boot from a Linux live CD or live USB, such as the Ubuntu live CD or the GParted live CD. Remove all the files outside
/home, then move the directories in
/home one level up, and remove the (now empty)
/home directory. (Of course, if there are files you want to keep outside
/home, make sure not to remove them, just move them to a good place!) Then use the disk/partition manager (I forget exactly what it's called), and shrink the existing Linux partition.
When you install Debian, tell it to use the free space as a partition to install the system. And tell it to mount your existing partition at
/home, without formatting it.
Did you test that your backup was readable and contained all the files it should?