Depends on how many users you are talking about. I've used sp_help_rev_login and reviewed the output script to select the users I was interested in. For one or two users I've scripted the logins from Management Studio.
This KB article has the code for the sp_help_rev_login stored procedure - http://support.microsoft.com/kb/918992
For SQL Server Logins remember to map the database user to a login afterwards. I've used sp_change_users_login in the past but see that Microsoft recommend using ALTER USER instead now.