*edit* You may or may not need your Windows XP CD or recovery CD from your computer manufacturer to complete this. Only insert the CD if prompted to do so.
Click on Start, go to control panel and click on add or remove programs. When that window opens, you will see on the left side "add or remove windows components." Click on it and wait for the next window to open. When it does, the window will show "accessories and utilities" as the first choice. If the check box is gray, uncheck that check box and recheck it. The check box should now be white and the size should be 18.0MB. Click next and it will install paint along with anything else that was left out. When it is done, just click finish. You should then be able to go to start, programs, accessories and find paint.