Specifies the programs that Winlogon runs when a user logs on. By
default, Winlogon runs Userinit.exe, which runs logon scripts,
reestablishes network connections, and then starts Explorer.exe, the
Windows user interface.
You can change the value of this entry to add or remove programs. For
example, to have a program run before the Windows Explorer user
interface starts, substitute the name of that program for Userinit.exe
in the value of this entry, then include instructions in that program to
start Userinit.exe. You might also want to substitute Explorer.exe for
Userinit.exe if you are working offline and are not using logon scripts.
[end quote]
http://www.microsoft.com/technet/prodtechnol/windows2000serv/reskit/regentry/12330.mspx?mfr=true
Some spyware and other pests can make use or exploit the userinit entry
to load their trash before the user GUI starts. If you remove spyware
and other such pests without removing them from the registry at the
above mentioned location you might experience the reboot behaviour such
as the one you are now experiencing. At other times the value may have
been completely removed, in which case you have to recreate it, the key
normally contains the following entry:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon
Value name: Userinit
Value data: C:\WINDOWS\system32\userinit.exe,
*Note the comma at the end of the value string*
Windows Log on and Log off immediately.
http://support.microsoft.com/kb/555648
At other times some of these pests simply highjack the userinit.exe file
itself to do their dirty deeds, they remove the valid userinit.exe file
and place their malware (renamed as userinit) at the valid location,
then there maybe no changes made to the registry value itself but when
the key is read and when userinit is executed the operating system is
actually executing the pest itself instead of the valid userinit.exe.
If your AV/Antispyware tools remove the impostor without placing the
valid file back to its original location then you may also experience
the reboot behaviour. See here for typical information on how to remove
invalid files and how to restore the registry value:
You cannot log on to Windows XP after you remove Wsaupdater.exe
http://support.microsoft.com/kb/892893
John