I rang Samsung Support to fix it, thought I may as well share. I use Windows 7 32 bit. I already had drivers downloaded and new version of Kies installed, but my computer still wasn't recognising my phone.
1. Connect phone to computer via USB
2. Go to My Computer > System Properties > Device Manager
3. Scroll down to "Universal Serial Bus controllers", and click on it
4. Right-click on the heading called "Samsung Android USB Composite Device"
5. Select "Uninstall", then, when prompted, also check the box that asks if you also want to delete the driver software
6. Click "OK"
7. Wait for uninstallation process to complete, then disconnect your phone from the USB
8. Reconnect your phone to the USB, and the drivers should automatically successfully install
