Single outlet resettable breaker?
Get a 20A to 15A converter cable, like this one: http://www.amazon.com/Conntek-1F515520-1-Foot-15-Amp-Adapter/dp/B00439KIF6



It is perfectly safe to plug a 20A device in to a 15A receptacle on a 15A breaker. If your device draws more than 15A, the breaker will trip. You can reset the breaker and try again, with a lower load. If you never exceed 15A draw, you'll be fine.

15A is a lot for a computer. You can use a Kill-A-Watt to measure your actual load, if you're curious.