Your left mouse button might be remapped or disabled. Try adjusting the configuration or resetting to defaults in the Xfce control panel, as described here:
http://docs.xfce.org/xfce/xfce4-settings/mouse
You can check the current mappings using the xinput command from the Terminal.
$ xinput list
⎡ Virtual core pointer id=2 [master pointer (3)]
⎜ ↳ Virtual core XTEST pointer id=4 [slave pointer (2)]
⎜ ↳ Mouseemu virtual mouse id=14 [slave pointer (2)]
⎜ ↳ bcm5974 id=11 [slave pointer (2)]
Look for your mouse in the list and note its id, presented as id=x; e.g., 11
The use xinput again to get the current button map:
$ xinput get-button-map 11
1 2 3 4 5 6 7
If the first number is "0", your mouse button 1 is disabled. Test re-enabling it with:
$ xinput set-button-map 11 1
Remember to replace "11" with the mouse's id from xinput list.
If this gets reset every time you login to your desktop, see if there's a file in your home directory called ".xinputrc" or a directory called .xinput.d/ containing files. Also see if there's an errant ".xinitrc" that might have xinput commands.
/etc/X11/xorg.conf? – Mitch Sep 5 '12 at 5:32sudo nano /etc/X11/xorg.conf– Mitch Sep 5 '12 at 5:47/etc/X11/xorg.conf. To create a new one usesudo Xorg :1 -configure– Noble P. Abraham Sep 9 '12 at 8:15