If malware infected winlogon, logonui, userinit, etc, it can lead to the
log-in issues you've described. It's also possible these files were
infected and smoked by your anti-virus application.
You could try booting into the recovery console, heading to \windows
and \windows\system32 and checking the date-stamp / file size of the
aforementioned files.... see if any appear to be infected or otherwise
altered. If you're able to find a replacement copy (in dllcache, update
directories, etc) you could try replacing the "bad" copies.
If it's simply a malware issue, I'd recommend remove the drive, slaving
it to another machine, and scanning the drive from there.
>> Stay informed about: *CANT log into Windows -please help