The first thing to check is to make sure you are not trying to log onto the domain. When you are at the login screen there should be a box under Password that says Log on to. You might have to click the Options button to get it to show. From there change it from the domain to COMPUTERNAME (this computer) where COMPUTERNAME is the name of the computer itself. That will log you on locally instead of on to the domain. You will have to have a local account on the laptop though to get in or know the administrator password.
If not then you can connect it to the domain and add a local user for yourself. This is important so you can log in locally after it's off the domain. Also be sure to make it a local administrator if you want full admin rights to the laptop. Then you can disjoin it from the domain by right clicking My Computer and choosing Properties. Then go to the Computer Name section and click the Change Settings. Under Member of click the Workgroup button and choose a name for your workgroup and click ok. You will need a domain admins name and password to confirm it. Then after a reboot it will not be part of the domain.