A user can be a member of multiple groups so that allows the user to have a combination of most permissible abilities. However, deny always overrides an allow so if a user is denied a permission in one group that overrides that permission in all his member groups. There are several built in groups like administrator's, backup operators, guest, network configuration, power users, remote desktop users and help users group. The name pretty much defines most of these groups.
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There are also network and interactive groups which differentiate on the basis of your location. Network group classifies users who log on using a network whereas interactive users are users who actually sit down at the machine to log on. Creating and managing user groups can be achieved through the Microsoft management console. This saves a lot of headache at the domain level since the domain administrator can create a domain level group in the domain environment. The local administrator can then add that domain level group into the local machine group he just created and this gives the members of that group immediate access to that machine.
Be sure to check if passwords are correct and caps lock is not turned on and also if your account has not been disabled. You can also turn on the guest account as a last resort to have limited access. This can be a security loop hole so most administrators avoid it. In a domain environment XP caches user log on information so you as an administrator can turn on a feature which prevents a user from logging on if the domain controller is down.
