If you had a USBtinyISP (or similar) then you could use that.
As you have a "Uno", you could use the "Arduino as ISP" example sketch and rig the Nano, as the target device, to the Uno. There aren't a lot of details of that specific (Nano-to-Uno) configuration, but there are examples using Uno/Duem. and "stand-alones", so you ought to be able to extrapolate based on that.http://arduino.cc/en/Tutorial/ArduinoISP
> > > Adding another useful link -- http://gammon.com.au/forum/?id=11637
BTW - Do NOT screw this up!