Why can't I connect my Samsung Galaxy 3 to my PC after installing the z4root app? I have a problem connecting my Samsung i5800 Galaxy 3 to a PC. First, I could connect to the PC. I had installed the z4root app in order to root my phone and then after installing it, I could not connect to the PC any more. 

The dialog to select how to connect to the PC does not appear, though the beep sounds when I connect the phone to the USB cabel. How do I fix this problem?
I had this problem too.  It was not related to any driver on the PC as it worked
and stopped working at some point.  The problem was on the Android phone which
for some reasons had some configuration problems.

To solve that, dial *#7284# on your phone so that it launches the PhoneUtil
application.  Then, switch to the USB -> PDA mode on the phone.  Connect the USB cable
(toggle USB -> Modem and then USB -> PDA modes if it does not work the first time)

The details are explained in:
How to repair the USB connection problem on Android Samsung phones