Depends on how you installed.
The first thing I'd do is gather up everything needed for going back to the old drivers. Then, kill X.
Ctrl - Alt - F1
If you run GDM or KDM kill those processes.
#fuser -v /dev/tty7
You shouldn't see anything about XFree86 running on that terminal.
#modprobe -r NVdriver
Now feel free to go back to the old driver. After you're finished check to make sure you have the driver build you want.
And lsmod to make sure its running.
#lsmod | grep NVdriver
Start X again in the fashion appropriate for your machine.