You installed using the Terminal, or did you Double Click on the Install Icon on the Desktop, the one that looks like a Disk?
Please check the Integrity of the Xubuntu Installation media with THIS TUTORIAL. If you got a bad download or Bad Burn Image it may cause problems.
You cannot Log Off and select another Distro, you have to shutdown/restart, and then choose the Distro you want from Grub2 Menu.
i can select wich Distro i use with the X gui log-in screen
You mean Grub2, does it look like the picture below, or similar?
You have more than one Distro Installed?
Please provide your computer model number.
Or, while in linux, run these commands in the Terminal. The first command installs "inxi" a tool we use to see our Computer Specs. Let it install, then run the second command.
sudo apt-get install inxi
1. Then Click “Edit” in upper Left of Terminal > Click “Select All” > Click “Edit” > Click “Copy” > Then in your Post, Click the "Code" Box at Top Middle of your Post, which looks like this “< >” and “Ctrl+V” Pasting into the Code Box, Or, just into the Body your next Post will be fine also.