[Xaccers]

If a computer account has been created in the domain you wish to log on to then all you'll have to do is type the domain name into the drop down list box.

For example:

Say your computer is already configured to log into a domain called alpha but you want to log into a domain called beta.
Assuming a computer account for your computer PC1 hasn't been created in beta, then you need to create it first.
This can either be done through the AD users and computers MMC from a machine in the beta domain.
Or, if you have sufficient rights, you can log into PC1 either locally or via alpha and go into the settings and add it to the beta domain.

I'm not sure, but if you're a domain admin, you may just be able to type in beta as the domain on PC1 at the logon screen, and it may then create the computer account in the domain for you.

Once a computer account has been made in the domain beta, you'll be able to log into either.
Your profiles will be seperate, one being c:\Documents and Settings\username and the other being c:\Documents and Settings\username.beta (unless you have a local account with the same username, in which case the alpha profile would be username.alpha, although if the local username account has been made after a domain username account has been logged into the profile will be username.PC1)