My S3 updated to 4.3 (locked bootloader) and I can't make it connect via USB, it won't connect. I've tried everything, every link I can find, reghacks, drivers (countless installs/etc). I know what I'm doing, I've been using computers longer than a lot of posters here have been alive.
The thing is the device isn't telling the PC what type of Hardware it is, let alone the PC having drivers. I actually think my phone is toast - I'm glad it still charges and you can (if you're not rooted) root via Towelroot without a PC.