Opening up your system and adding Crucial upgrades should in NO WAY void your original warranty.
Nearly every major computer manufacturer, including Dell, IBM, Gateway, Apple, and more, installs Micron parts as original equipment. Micron works closely with these manufacturers to create the highest-quality products for both their systems and yours. As a division of Micron, Crucial guarantees that the upgrades you buy will be 100 percent compatible with your system, or we'll refund the price of the incompatible part.
That's great news in more ways than one. Not only do you save money buying Crucial upgrades direct from the factory, you often also get the same components built into today's leading machines.
More information on this, as well as links to the full text of some manufacturer warranties, is available here.
In the US specifically, the Magnuson-Moss Act, enacted into law by the US Congress in 1975, protects consumers' right to use the products they want.
How does it work? In legal terms, the Act prohibits manufacturers from requiring consumers to use products or services from a particular manufacturer in order to keep their original product warranty valid. That means, in most cases, you don't have to upgrade with your system's original parts using components from another manufacturer specified by your system manufacturer to maintain your system warranty.
In much simpler terms, you can add Crucial RAM or Solid State Drives and your system warranty will remain valid!