The drivers are on the CD you got with the phone. after the drivers were installed choose "Phonemode" while you plugin the phone. Then open MyPhoneExplorer an go to Menu-file-settings-search device
Download the SonyEricsson-Suite for the K770 from the SE-Homepage. Try to extract the exe and look, if you can find drivers in it.
If not, you can also install the Suite. The drivers will install with the suite.