There are 2 ways to do this depending on how comfortable you are.
1) User Windows Easy Transfer under Programs -> System Tools.
This method is very easy,
Login as Local user and use the Easy Transfer program to create an Easy Transfer File
Put the machine on the domain.
Login as Domain user and run the Easy Transfer program to unpack the file
The problem with this method is it involves a copy and depending on how much data they have that can take a while. So what's the fastest way to do it?
2) Move the files from the local user profile to the domain user profile.
Using a move operation in Windows 7 as opposed to a copy is much faster if you are moving between locations on the same hard disk.
The first time you log in with a user a profile is created for them under C:\Users The profile for each user is simply a folder in this directory with their username. If you are using the same user names the new folder will be called "username"."yourdomain". So all you have to do is move all the important files from the old profile to the new profile. Now technically you could just do a select all and move literally all the files from one profile to the other, but I would not suggest it as there are system files that can make things a bit wonky if they don't move correctly.
Most of the good stuff will be in the folders Documents, Favorites (assuming you run IE), Pictures, Video, Music and AppData\Local\Microsoft\Outlook (Appdata is hidden by default) if you have outlook with .PSTs
Best Answer
The standard behaviour in an AD domain is to automatically add the domain group "Domain Users" to the local group "Users" on each computer that is joined to the domain; this allows all members of "Domain Users" (which by default contains all user accounts in the domain) to log on to any computer in the domain.
If you have a non-standard setup which doesn't allow all users to log on to all computers, this can be due to two reasons: either your AD user accounts were removed from the "Domain Users" domain group, or that group is not member of the local "Users" group con your computers. You should check these two settings and fix them if they are not right.