Windows can't check the Licence for this computer

After 2 years of no problems... now when my Sony computer
starts up and goes to the welcome page, after I click my
name this error msg apears:
Window product activation
A problem is preventing windows for accurately checking
the licence for this computer. Error code 0x80004005
Then it logs off and shuts down!
I have always downloaded the patches and updates. And I
can't seem to get any help from anyone...PLEASE some one
here help me
Thanks, Alan