Sad to say, this is one symptom of an LCD panel that has gone bad. Failure of some of the panel's internal circuitry is allowing the affected columns to remain on all the time, even though the display is otherwise working properly. There is no way to repair the panel when this happens, so all you can do is replace it. You can continue to use the computer if you don't want to put the time and money into a repair.
If you do want to try replacing the display panel, it's not a difficult repair. It just means taking out a lot of small screws. You can download the
maintenance and service guide from HP, which covers the process. It also lists part numbers for the display so you can search for the one you need. (Two different parts are listed; you will need to look at the sticker on the back of the display panel to see which is used in your computer.) Replacement displays are available from several online sellers, and eBay is a good place to start looking. I found several listed for $100 or so, and if you're not in the US you should be able to find a seller who can ship to you.
Let cost decide what you should do. If you can live with the lines, it costs nothing. Otherwise, you can find the cost of a replacement panel and weigh it against the cost of a new computer. You may decide it's worth repairing your current computer, or you may opt for something newer with more features.
If you do decide to fix what you have, you won't need to deal with reinstalling software on a new computer, transferring data, etc. There's also a good feeling that comes with doing it yourself. Whatever you decide, good luck and thanks for using Fixya!