Are you not seeing a box to tick to Activate Windows Now? If not, you may see a tray icon offering you three or thirty days to activate. Don't believe the 30 - do it now. If the system can't do it automatically, use the telephone method and quote the number printed on the CoA on your computer case. If they say that's already in use and if you don't have a reason to be installing it again, you will need to buy another copy.
If you aren't seeing the message but you have the icon, force the automatic activation process to start. Go from Start to Run and type regedit into the box then press Enter. Navigate to H_KEY_LOCAL_MACHINE>Software>Microsoft>WindowsNT>Current Version>WPAEvents.. In the right hand pane, double click OOBETimer, click Modify then change at least one digit of this value to deactivate Windows. Restart the machine and the error the system finds should start the process running.
If that fails top make teh difference, click to Run again and into the Open box, type the following command, and then press Enter
In the next form select "I want to telephone a customer service representative to activate Windows", then click Next. Click Change Product key then type the Product Key from the CoA on the case and click Update.