New user profile method

A relatively elementary question but I wanted clarity directly from some experts.


Some computers joined to a small domain are running slow. Yes I can run some utility programs to check for viruses,disk and registr cleanup etc.  but if I need to just creat a new profile.  Now each machine is only used by a single user and they do not download and install programs but they have been using same machine for a long time. If I wanted to create a new profile for them, is it best to log on as admin and rename their profile and have them relog on thus creating a profile? If I create a new profile beforehand and copy all files from old profile( instead of the system files ntuser.dat,etc.), how do make sure they will log onto the right profile given they logon onto domain and  current  one is say crummy.domain on docs and settings. I am not sure which is best. Also, Can I revert back to their old profile if some reason I need to and how?