I know it has something to do with nVidia or libGL, it doesn't happen if I use vesa.
It seemed to have started to happen since I installed XFCE + xfce4-power-manager, it started to hang when I tried to suspend, log out, or shut down. Not only on XFCE, but even on KDE and BlackBox. After I uninstalled, it kept happening, maybe it had something to do with reinstalling the nVidia driver over and over every time I recompiled my kernel modules because make modules_install removed them all (I use nvidia's propietary linux installers)
It just freezes my computer every time X is stopped for anything or when I switch to a tty, but NOT with Vesa. If I push the caps lock key, the bulb doesn't turn on/off...
I tried using the default xorg.conf that comes with slackware, but changing the default driver to "nvidia". It does exactly the same theing it does after installing with nVidia's linux installer, it looks high res, if I try to switch to a tty it freezes.
Please help It was working perfectly until a while ago...

Not an expert on this, but did you try making nvidia setup X for you? You should be able to do this with nvidia-xconfig or something similar. It should be available in the nvida-utils package. Personally I never use nVidia's linux installers because they have given me nothing but headaches. With practically any Linux distro, you should try whatever drivers your distro provides. Only when you can't get those drivers to work should you use a third party installer.

Thank you, it's over now... I had to recompile the kernel (SERIOUSLY, just recompile the damn kernel) re-installing the modules didn't seem to be enough. Really damn weird.
My blackbox and KDE sessions were destroyed too...