1. Get into Safe Mode. If access through installation medium is not possible, shutdown during boot for 3 times. Go in troubleshoot/advanced/start up something
2. Win+R „regedit“ and go to
HKEY_LOCAL_MACHINE\\SOFTWARE\\Microsoft\\Windows NT\\CurrentVersion\\ProfileList
3. Expand ProfileList and chose the profile with ProfileImagePath = […]systemprofile
4. Set RefCount from 1 to 0
5. Reboot.
Pray.