Maybe this will help you thanks, to http://android.stackexchange.com/questions/4322/my-samsung-galaxy-3-gt-i5801-is-not-being-detected-in-my-pc-what-should-i-do:

 1. Unplug the USB cable
 2. On the cell phone, dial the following number: *#7284# Once the last
    # is hit, the PhoneUtil application is launched. Choose USB -> Modem and then USB -> PDA mode. The good mode should be PDA. Even if the
    mode is PDA, switch to Modem and then back to PDA.
 3. Plug the USB cable.