after doing some researching on this topic it seams to me that is is common but not very fixable
i would suggest running ccleaner (
http://www.ccleaner.com/ )(free) and malwarebytes anti-malware (
http://www.download.com/Malwarebytes-Anti-...&tag=button ) (use the free version, works very well)
these two programs should clean up just about all of your computer files, and hopefully it could help your problem (if its a malware problem)
-joey
UPDATE:
i found this on another forum:
MOD EDIT: Backup your registry before attempting any modifications. Even minor mistakes can make your system unbootable.
"open registry (menu Start -> run (or run Comand prompt): regedit).
open: HKEY_LOCAL_MACHINE \ SOFTWARE \ Microsoft \ Windows NT \ Current Version \ Winlogon
At right pane there is "Shell". The value of it must be "explorer.exe" (and nothing more - in my case there was activedebugger.exe too, when I removed it explorer start appear on startup)."
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i would try scanning first and then try that
good luck