How do you get your computer to quit lagging?

First, go to your interface and see how high you have your video quality. Set everything to low and play. Usually, this will fix lag problems if the server you're on is crowded. If you have a bad computer, whether it's because of viruses, it's old, etc. update your computer to Windows 7. If you don't want to update, then transfer everything you want to keep on the computer onto an external hardrive and begin recovering your computers original settings.