CTRL-ALT-BkSp will reset the x server and dump you back at your login prompt or screen. you say -
But than he log out ROOT and log in my own acount...
quick digression -
this suggests that you've logged in as root... *don't do this*!!! unless you're absolutely certain you know what you're doing!! you should only go root when absolutely necessary and / or prompted by your system.
anyway, regarding your question, for the purposes of installing the nvidia drivers..
back at the login screen, you should see an option to log in as 'failsafe' or similar - select this option and you should log in to a console, from where you can install your drivers. there may be some post-install issues to deal with depending on your distro - read the docs. i found the nvidia driver installation relatively straightforward (certainly compared to ati, which can be a nightmare).