Keep getting logged out

I have just installed wordpress, and a theme. But then i keep getting logged out every second. I can do any changes or anything else, in a second i get logged out, just type password get logged in a can se the controlpanel and im logged out again.

Have tryed to delete coockies close all and start over, but same happens all the time. Using Safari, Firefox but same happens. It is impossible to admin the site?

Did installation 1 may.

Hope someone knows why
regards