I know why it's like that there are many possible reasons
1st : your plug are too shabby or if it's a wireless keyboard, batteries are down.
2nd : Try go go to Start menu / Control Panel / Ease of Access / Keyboard and uncheck all that is checked, then restart
3d : try using a (USB) keyboard if you can't afford it, when you start your computer press all the keys on the keyboard but make sure you press the following first : Num , Lock Caps Lock , Scroll Lock then try to press as much as possible util the Welcome screen pops up.
4th : If nothing still work you can still use the Visual keyboard (virtual keyboard or OSK / On Screen Keyboard) : Start button / All Programs / Accessories / Ease of Access / On-Screen Keyboard.
(Hope it helps)