thease thing happens due to a ur fault or unauthorized softwears and also can b a virus infection too.
try this,,,,
when the login screen came hit Ctrl+Alt+Del and then in users tab type "Administrator" (without quotation marks)and leave blank 4 da passwrd feild and then hit Enter....
And now you will be log in to a hidden administrator account..then frm there u can restor ur pc,or you can create a new user and use that....
(if you a r nt directed 2 a loging acreen then u can goto taskmanager and select log off..and now you'll b in login screen....)
bu da way if this s a virus infection may b this methed will not work(may be,,who knows


)...but as you said i thing it's because of the some corrupt in ur explorer.exe .......try it