On a desktop, or any complete keyboard, the number lock function is pretty cut-and-dry. You turn on the number lock to use the number pad as numbers, and turn it off to use the pad as arrows. Since laptop keyboards don't have a number pad, it gets a little more complicated--if the number lock is on, certain letter keys can't be used. Hewlett-Packard further complicated this problem by making the number lock button a secondary function, so the button is difficult to find. Luckily, if you can't find the button, or your HP laptop doesn't have one, you can use the Windows integrated on-screen keyboard to turn the number lock on and off.
Press and hold the "Fn" button. The button should be located in the lower left side of the keyboard, next to the Windows button, and is blue.
Press the button labelled "Scroll." It will be on the top row, and should be the fourth button from the right. In a blue box under "Scroll" it should read "Num lk." If your model of HP laptop has a number lock light, it will now turn on.
Release the "Fn" button and try typing the letter "J." If a "J" comes up instead of the number "1," your number lock is now off.
Click on the "Start" button on the bottom left side of the screen.
Click on "All Programs," then move your mouse over "Accessories" followed by "Accessibility" and finally click on "On-Screen Keyboard." If you use Vista or a later operating system, you can type "On screen keyboard" into the search function and then click on the program when it appears.
Click on the number lock button, shown as "NLK" just above the number pad on the right side of your on-screen keyboard. When the button turns grey, the number lock is off.