|Try plugging in an external keyboard and typing with that. If the characters are fine with that, you need to replace the built in keyboard.|
Laptop keyboards are a frequently replaced item and relatively inexpensive to buy on the web. You need the part number for it in order to find the right one.
HP has a maintenance manual available for most laptops for the model or model series, that you can look at online or download - in that you can find out how to remove the keyboard - sometimes you can remove it without having to open up the base of the computer - the part number label is probably on the bottom of the keyboard.