Rebooting clears out the modules in your kernel. It clears memory; therefore, it must effectively unload all modules.
The problem is that you need to modprobe (or insmod; with this module, they do the same thing) the nvidia module, then, without rebooting
If it still doesn't work, then look in syslog (or the output of dmesg) for the reason why. More often than not, it's because you need to disable PnP OS, or enable Assign IRQ to VGA, in your BIOS. But most likely, the problem is that after you reboot, the module still isn't loaded.