[Hampshire] X problem - I think

I wonder if anyone could help me with a problem - I'm not sure how to
trace what's going wrong.

I'm using Debian Lenny with xfce as my default desktop. Recently a
problem has started to occur where I lose the inability to input from
the keyboard. The pointer still moves with the mouse, but I can't click
on anything either - nothing obvious triggers it. I can switch to a
terminal using alt+SysRq+R, from where I can kill the desktop and start
again. I've tried using Gnome, but the same problem occurs. It also
happens to my boys, who use xfce, but hasn't occurred yet when I use
WindowMaker.

As usual, any suggestions gratefully received.

Owain Clarke