Kies and the Samsung drivers have some issues with certain computer chipsets. NVIDIA chipsets are a bit of a hit and miss, where you CAN get things working, but you may run into a number of problems. There are others that also have issues.
So, here is the checklist of things to check for prior to doing the JB update:
1) Uninstall any antivirus on the phone
2) turn off any firewall software on the computer
3) If you have problems with the connection, use the USB ports on the BACK of the computer, or on the other side of the laptop. USB ports tend to come in sets, and some may work while others do not.
4) Make sure you are using Kies from an account with administrator permissions. You don't have to be on as the Administrator account, but you can't be using a limited or standard user account.
Check your cable, is it the original that came with your phone? Try a different one.
Try those and see if they help.