I've seen this problem before after recovering from a virus. The problem was related to a registry setting that the virus had modified. Removing the setting caused the computer to loop back to the login screen when trying to log in. Do you have a list of items the workstation was infected with? It could help narrow down which registry key to check.
Check the keys located here:
The following two keys should contain these values:
Userinit = C:\windows\system32\userinit.exe,
If the computer is on the network, run regedit from another machine and open up this computer's registry to modify those values if they're incorrect.