Just had a customers machine that was heavily infected with malware. After taking care of the malware it would log in and log off. This paticular malware did not change the HKLM\Software\Microsoft\Windows NT\CurrentVersion\Windows\WinLogon C:\WINDOWS\System32\userint.exe, line. It had deleted userint.exe from C:\WINDOWS\System32 folder. I resolved the issue by copying userinit.exe from C:\I386 folder into C:\WINDOWS\System32. now it boots fine.