Hello and welcome to Just Answer.
You said " My 2000 buick century won't start. everything seems to work(lights,horn,dashlights,etc) except the doors locks, power window and radio."
The vehicle antitheft system , door locks and power windows are all controlled by the BODY CONTROL COMPUTER (BCM). If there is a malfunction in the BCM or its circuits you could end up with the no start condition you have described.
Unless you have significant experience diagnosing electrical problem, I'd suggest that you need to enlist the help of a repair shop that has access to a tech2 scantool and the services of a good electrical tech to do the necessary diagnostics on this system. If you have some experience, read on.
If you have access to a testlight we could do some basic checks to see if the ignition switch is simply malfunctioning and causing this condition.
You need to connect the clip on the testlight to a good ground and then probe the PCM BCM U/H RELAY 10amp fuse while turning the key on and cranking the engine. If this fuse does not show a good bright light, the ignition switch is the likely culprit. Try this out and get back to me if you wish.
Does this answer your question ?