I've been tempted to switch back from Unity to KDE for several days.
So, today I decided to actually do it.
Everything was fine, then, at some point, after setting up everything, I restarted and...
Wow. For those who haven't seen it, it kind of scares. The numlock and capslock leds on the keyboard keep blinking and nothing else works.
I rebooted again. Same thing. Then again. Sometimes, it was even a hard freeze: not even the blinking leds.
All this happened right after entering the login password in KDM.
Then, I just decided to reinstall everything (I had just installed kubuntu-desktop and uninstalled everything from gnome, as explained here.
After reinstalling, upgrading, configuring... Guess what?
The same thing.
Then I just realized it was when connecting to the wireless network that the system froze.
So, my guess was the wireless card driver. Bingo!!!
I have a Dell Vostro 3300, which comes with a Broadcom BCM4313 board. The default driver for it is brcm80211.
I had seen several days before in the Hardware Drivers program that another driver was available for it: Broadcom STA driver.
As I had nothing to loose, I just tried, and it worked! \o/
So, here is my tip if anyone encounters a hard freeze or a kernel panic and has the same hardware: install the Broadcom STA driver!
Just as a note, neither KDE nor Kubuntu are to blame here: it was the wireless driver's fault.