Via Microsoft due to possible virus infection, try this:
Edit these values and type the correct path of shell :
Shell = explorer.exe
NOTE: These files may also be deleted by spywares. You may need to extract them using Windows CD.
Steps for rectifying this problem:
Log on to a networked computer.
Point your cursor to HKEY_LOCAL_MACHINE
Select File > Connect Remote Registry
Type computer name (infected computer)
Navigate to the following location in registry of destination or infected computer
Edit these two values in right pane:
Change these two values to
Userinit = x:\windows\system32\userinit.exe
Exit from Registry
Restart Infected computer.
You should be able to log on to computer.
There might be possible problems in you registry files or the system files of your Windows Server 2003 software. If you are seeing this errors frequently, you really need to backup your important files to another storage device this is just a precautionary measure.