severe security problem-cannot add/remove applications

Cannot access audio, video on anything. Problem(no video, some audio) has existed for some time but ignored it until the latest (no audio or video access). Cannot rectify, add/remove will not load applications - ie gnash flash viewer, gstreamer. I have neighbours who have spliced into my cable line and have a mirror screen of mine and control as well. This has been a problem for some time as mentioned but they are slowly taking everything I was able to do away. I'm not sure if theirs is the computer that has been eating away at mine or not. Hope you can come to my rescue or at least put me on the path to solving this problem once and for all. It may help to know that these freaks pestering me are of the ilk of the NWO.