I am rebuilding my slack machine from scratch with slackware 10.1.
During the install I select xfce as the default window manager.
I'm reinstalling for the third time now (and I plan to pay closer attention this time) but in each of the other 2 installs, after I started X, I had a nice xfce screen and a nice right click menu with settings and apps and all that neat stuff.

Then I go download some stuff, nvidia, acrobat, java, firefox, etc. And then, as if I just popped into the twightlight zone, I notice I no longer have the xfce logo in the midle of the screen. The handy little right click menu is no longer there. It's now just a little abreviated menu with no apps or settings, etc. And I haven't installed anything yet, except the nvidia driver and firefox.

It's like a basic gnome window, or something. I don't know where it changed, but I hope to find out this time through. After I noticed the change I launched xwmconfig and it still had xfce selected.

Does this sound familiar to anyone?

That reminds me when I launched nautilus without passing proper parameters and it took over all my "desktop" (background, placed icons and had gnome's right click on desktop).
But I don't see you mention running any program so I really don't have a clue...

Yes, I also think that it could be related to nautilus. Run nautilus with --no-desktop option to prevent it from changing the desktop. Also, if XFCE desktop is not there, run
xfdesktop
When you quit your session, have the settings saved (it's the default option), and in the next login xfdesktop should start.