Learn to build your own unit. As others have said about a dozen times so far, you save so much money, and learn so much. I'm in the process of building my own unit, because the last time I bought a PC pre-built, I was sorely dissatisfied. Bought an Acer Aspire desktop. Don't get me wrong, I love Acer, and for anyone completely unwilling to learn to build their own, its awesome. Sadly, the salesman will say anything to get you to buy. Got the unit home, tried to run the game I had hoped to play---Nada, even after having been told that the system would play it. Did my research. Learned a mere month after my purchase that NOTHING was right. Couldn't OC my e4500, a proc that is known to reach 3.0 ghz easily. Had a to-go GPU, meaning I couldn't do anything in the way of gaming. In short, I had paid $500 bucks, for a unit that I couldn't upgrade well, and wouldn't do what I wanted.
I'm building my own now, with an intent on being able to do certain things, certain ways. I get to pick my own case, my own cooler, fans, HDs, everything, and its cheaper than what I'd pay for a unit that can compare. In addition, building a PC is relatively simple. As for warranties, if you buy new, half yours parts COME with warranties of some kind.
Now, if only I could pick my parts XD