disabling computer accounts?

In AD< what does disabling a computer account do?  I think we have users who are logging in with disbaled computer accounts and its working. Disabling user accounts always prevents them from logging in though


Also, how do you set a policy, that says something like "cannot add this computer to the domain unless it exists as an object in the domain already."  That way, we have to add the PC to AD before joining it to a domain.