Problem with my Acer Extensa 5220

Hello,
When I start my Acer Extensa 5220 laptop it displays the following message...'This copy of Windows must be activated with Microsoft before you can log on. Do you want to activate Windows now.'
When I click 'Yes' it goes to a new window and tells me ...Windows is already activate..and then proceeds to log off.
How can I fix this problem?????
Any help would be greatly appreciated as its been like this for three days now and I really need to get into it for Work.......
Thanks