I have had a lot of striping in the past with the same machine.
You have to do some testing.
FIRST. borrow a monitor from a friend and see if the same problem occurs . Use their vga cord for the connection. This will eliminate two could be problems. The monitor and the cord. If the monitor looks good then use your cord. Now if good it is your monitor, if bad with your cord it is your cord.
SECOND. Take your monitor to a friends and hook it to their computer. If the monitor is good with your cord at the friends then it is your video card
THIRD. In the first test, if you have the same monitor trouble all around then it is your Video.
NOTE: You did not say if this machine had an onboard video or the video was a separate card.
If a separate card and the tests prove it is a card then replace the card. If you discover the video is bad and it has an onboard video then try a separate video card for the fix.
A computer shop would probably give you a AGP video card to test. The picture is of a DELL and you say it is running windows XP. I would say to watch out for cost as on ebay you can get a much newer computer that works for less then the charge to have a repair shop do any work.
I again can only guess at the AGP slot on the motherboard. Some came with no AGP slots just ISA.
So to summarize. You are testing the monitor, you are testing the monitor cable, then you are testing the video. Do not spend money on a repair shop be aware of the cost and get a quote. Your computer is not worth $100. Almost any friend can remove your hard drive, stick it in your newer machine and you can retrieve your data.
Way good luck there