Click on your Start button, right-click Computer, left-click Properties. At the bottom of the System window that pops up, you'll see Windows' activation status and be given the option to change your product key. As long as the product key on your computer matches the copy of Windows you've installed, it should activate and you should be good to go, but if you've installed a version of Windows that does not match your product key (for example, trying to activate a Professional install with a Home Premium key) you'll need to replace the incorrect version of Windows with the correct version.
Worst case scenario, if your computer was manufacturer built, you can contact them for a set of recovery disks for your system, which will activate Windows automatically.
Hope that helps!
