I had two problems with USB connection
1. USB connection not detected by the phone- I cleaned microUSB connector of phone using alcohol and toothbrush 3-4 times after some gap. This removed the corrosion of microUSB port pins and the phone sharted detecting USB cooncetion as well got detcted properly by Windows Vista as "Samsung Composite device" (and not as unknown device). At this point Galaxy S started getting recgnised and mouted and USB drive in " mass storage mode". Thanks to this forum which mentios microUSB corrosion in several posts.
2. However, Still Kies was not able to make connection to phone in "kies mode". The "connectiing" icon use to keep rotating endlessly". After 3 hours of troubleshooting including reinstall, reinstall drivers, removing files with log file name extensions, I found that Windows defender (antimalware protection) service was slowing the detection of the phone. I went to control panel -> classic view-> windows defender and disabled the real time scan and rebooted. This resolved the issue.
Please check if Windows defender or any other realtime antivirus protection is causing the malfunction.
with best wishes,