Does installing a [component] void my warranty?

In the United States, it does not do so automatically. According to the Magnuson-Moss Warranty Act, the Federal Trade Commission (FTC) has made it illegal for modifications to automatically void your warranty.  The important point is that the dealer has to be able todemonstrate that the modification was the cause of the failure.  More information can be found at the Specialty Equipment Marketing Association (SEMA) site and the Diesel Injection Service page.  You will also want to search the archives for posts by Scott Borders and Jeff Fritz for important highlights of the Act.

However, some dealers are not keen on modified vehicles and may claim that the problem is somehow related to the vehicle modifications.  Plus, it is usually better to avoid the entire mess rather than enter an antagonistic relationship with your dealer.  Most people recommend talking to the service managers and finding out which ones dislike modifications, and which ones don't mind.  Other people install modifications that can be easily reversed, so the car can revert back to stock for each trip to the dealer.

Sometimes, however, you will find yourself in a serious dispute with the dealer. Should it come to this, you will probably need the phone number for either the FTC or SEMA. The FTC, on request, will send out legal information regarding warranty claims. SEMA is actively promoting the MMW Act; more information can be had from SEMA's Steps To Take if Your Warranty Claim is Denied page.

[Tip: you would have already found the SEMA FAQ if you had checked the FAQ Locator.

For audio or electronics enthusiasts, there is also a < href="http://www.cemacity.org/gazette/files/knyrights.htm" TARGET="_top">warranty information page from the Consumer Electronics Manufacturer's Association (CEMA). This page deals specifically with aftermarket mobile electronics and their effect (or, rather, their non-effect) on automotive warranties.

Also keep in mind that different states have different emissions laws in place.  By modifying a component that may affect emissions (read: everything engine-related) you may have difficulties with smog checks.

Members outside of the United States may have similar legislation in place; you will have to check with your local authorities for details.

The Last Word: Warranty? What warranty?