I have the same as you version of Windows and have the same problem. Only workeround for now for me is: after connecting phone to PC I am going to Device manager and using search for changes in devices option few times sometimes it work. If not I am using uninstall driver for MTP in mobile devices section, reconect phone and typicaly it working if not need do it again. I saw on other forums people have same problem with Xperia S but no one found 100% working solution.
Sent from Sony Xperia S
ArekS | Xperia S | Xperia Arc | Nexus One | Xperia X10 mini