Kies is known for it's overall crappyness. I have the same problem with my sgs, like most sgs owners. Kies air works for me, its free in Samsung apps. But it is very slow and you can't use it to update android. The problem could also be that the previous owner rooted the phone. Search the market for an application to find out if it's rooted. Sometimes it's possible to unroot, you can find tutorials for that if you Google "unroot sgs ", add the version of android to your searchterm for more relevant results.