Have you installed the drivers or Samsung Kies?

guys,

i always get this error when i connect my S3 to my pc...the pc can't detect my s3 at all...what's wrong?

Originally Posted by Hasse Steel 
My Galaxy S2 could not be detected on either Windows 7 or Odin.Had upgraded Android 2.3.4 after rooting.
Found the tips below on forum.xda-developers.com/showthread.php?t=1317916

That finally did the job for me! Many thanks to Ashtwinks!

1. When the phone is upgraded to Android 2.3.4 and above (mine is 2.3.6) Kies stops recognizing the phones and USB connection / Serial port connections etc get reset on the phone
2. Even if proper drivers are there on the laptop (mine is Windows 7 64bit) they are not recognized
3. Device Manager shows "unknown device"
4. Updating the driver also does not result in any help
5. solution lies in the phone
a. key in the dialer *#7284#
b. it has 4 option there..
UART (MODEM/PDA)
USB (MODEM/PDA)
c. For USB. select the PDA.if it already selected to PDA. reselect it by select MODEM and then PDA again.. Only change the USB setting.. andthen press back. try reconnect (thanks to napi1 @ androidforums.com/samsung-galaxy-s2-international/447145-sgs2-kies-not-syncing-os-2-3-5-a.html#post3484588)
6. The Laptop will start recognizing the required drivers and install them
also
Phone is not recognized by laptop when it is in USB debug mode, MyPhoneExplorer does not recognize when it is not in Debug mode