Since you tried different computers we call rule out the USB port on the computer side as the problem.
That leaves:
- USB cable
- Settings on camera
- connector on camera
- fault in camera
If you check the settings and use a new cable and still have the problem that would leave 3 and 4. Unfortunately having the camera serviced is probably the only solution for these. Given the age of the camera and lack of parts it would probably be more economical to replace the camera.
Edit: If do upgrade your camera take note the PKTether will support up to the K-5. The current cameras will work with PentaxForums' program. As I recall the K-30/50 are not supported by any of the tethering programs. Someone will correct me if I am wrong
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The PKTether site appears to be gone but a member or two here have archived the download file. I can't get the Pentax remote utility to load on anything above Windows XP.