First consider new items on the market: tablets like xoom, archos, playbook and ipad, or the various linux netbooks that are nearly impervious to viruses. For some situations, a desktop or laptop is not the best choice for environmental, battery life and other concerns.
Always look up reviews of a product before you buy.
If purchasing through a store, print competitors pricing lists from their websites before shopping.
Consider alternate places to purchase such as ebay, dell off lease action, microcenter (local store, off lease options)
If its mobile, get an extended warrenty if you can.
For really young kids we recommend linux or mac products as they are harder to infect. Linux has a nice balance of price and security, and most kids can figure it out.