When XFCE starts, all windows of the previous session open at once (on the same desktop) and stack on the top of the screen. The top bar is not shown and windows do not respond normally. XFCE was therefore impossible to use. Alt-tab switch does not work to change window.
I removed all files in $HOME/.cache/sessions from Ctrl-alt-F1 text terminal, what got XFCE back into work after a logout/login.
I got the same problem on two different machines. On both machines, Ubuntu 11.10 was installed and XFCE was installed using "apt-get install xfce4", what installed the 4.8 release of XFCE.
Sounds like xfwm4 is not running
Likely to be a duplicate of bug #5379 which was fixed a while ago.
*** This bug has been marked as a duplicate of bug 5379 ***