|"I thought there was something wrong with my computer, so I went an bought another one"|
That was your solution? Buy a new PC?
Why are you using the VGA interface? You should be using digital not analog. Doesn't your computer have DVI or HDMI outputs? Doesn't your TV have DVI or HDMI inputs? Is the TV being used as the one & only montior or do you have dual displays? I'm guessing the TV resolution should be 1920x1080 @ 60Hz.
Other than that, there are settings in the PC graphics software menu & TV menu. We have no idea which JVC TV or computer you have so it's difficult to tell you where to look & what to look for, but it seems odd that a new PC would be running Vista?