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.