Comment 10 for bug 216908

I take no credit for this - I read it on one of the Ubuntu forums - but it solved this problem on both my Dell machines that were suffering from it.

1. Go to System - Administration - Login Window.

2. Under 'General', press "Edit commands ...'

3. Select 'Halt command' - in 'Path' box, cut the text in quotes (I.e. "Shut down via gdm") - press "Apply command change" - paste the text back - press 'Apply command change'

4. Select 'Reboot command' and do as for 3. above

Try restarting a couple of times and see if this has fixed the problem. It seems to have done so for a number of people, including me - although I wouldn't care to have to explain exactly how it's fixed the problem!