I had a power supply go out in my home computer that I got last Christmas and through some general troubleshooting steps with Dell Technical Support over their chat option, we were able to determine the problem and they are sending me a new power supply. All I needed to do was to enter the Service Tag number from the side of my case and they were able to determine the length of my warranty and help me from there. It took a few emails back and forth to confirm some tests and my new parts were already shipped.
Actually, there's a funny little story about the length of my warranty. At first one of the tech reps emailed me back and typed something to the effect of...
Your warranty has already expired as of December 18th, 2008.
Considering that this date is still 3 ½ months away, I wrote back politely that of course the problem would be covered. The tech promptly and humbly apologized for the confusion. I really think that he meant to type "will expire" as opposed to "already". Simple mistake.