My partners family have a Dell Tower, has to be around 2 - 2 1/2 years old, same problem.
When you boot it all you get is the Dell Boot Screen, every now and again it boots into windows, same was advised, the motherboard, but after taking the thing to peices, removing half a ton of dust and putting it all back it seems to work ok if its left on its side, if you put it upright it just dies, so i am guessing a loose connection or similar...My Emachine tower suffered a similar fate, but that was ram related...
If its out of warrenty they;ll not want to know unless you are willing to pay, systems are so cheap, its probibaly worth just getting another one, 17 months is a little poor, but i'v dealt with HP computers going dead just after the warranty is up...