The Zhongguancun area is sometimes called China's Silicon Valley, because of its many universities and IT-based companies. It also has a score of really big markets, selling anything you can imagine in computers and electronics, and they are all within walking distance from each other.
One market that stands out in the crowd is Zhonghai electronic market, selling used computer stuff and notebooks, which could be convenient if you prefer low prices rather than getting the latest model.
The markets in this area really have anything you could possibly need for you computer. What is more, many of the markets offer cheap computer repair service.
Salespeople and atmosphere
Few of the salespeople speak English. If you find someone who does, however, they are often able to help you even if their store doesn't have what you are looking for. If you are planning to buy something a little more expensive, and don't speak mandarin, it can be wise to bring someone who does. Prices are negotiable.