Are you connecting your phone directly to your PC without a charging dock?
If yes, check so you have the latest version off Windows Media Player installed and the latest Service Pack on your PC. If the problem still persist you can check so the drivers are installed correctly by looking in the device manager. You can find the device manager if you right click on My computer > Manage > Device manager.
In the device manager you should find your phone, if the drivers are not installed correctly there should be an yellow exclamation mark on the device. Right click on your device and choose "Uninstall drivers". Disconnect the phone and connect it to another USB-port.
Still not working? Check our FAQ for further troubleshooting:
http://talk.sonymobile.com/t5/FAQ/USB-connection-problems/m-p/348074#U348074
