Windows and other Microsoft programs like Office will run for a while without being activated / registered, but then they will stop and insist that you activate them. You should have an activation key on a label on the back, side or bottom of your computer. If you activate Windows and type in this key exactly, Windows or Office should now continue working. Microsoft Activation FAQ
Occasionally the online activation fails, or tells you that Windows is not genuine. In that case, call your nearest Microsoft Activation centre and they can manually activate it over the phone.