|A HP Pavilion can be a laptop (or netbook ?), or a desktop computer. |
If your computer is a laptop or netbook and if it's been used a lot, your problem is common. An external monitor should work fine, but your built in display may be starting to fail.
If your computer is a desktop computer, try connecting a different monitor, or connecting your monitor to a different computer.
I'm assuming that you have NOT changed the resolution of the display in Windows.
If you have a laptop or netbook, or a desktop computer that has an LCD monitor connected to it, LCD displays have one "Optimal" or Native" resolution specified for them at which the display looks the best. All other resolutions will not look as good, the most noticable thing being the text on the screen will NOT be as clear, and the display of the video may be stretched horizontally or vertically, or have black areas on both sides or on the top and bottom,