There has been an emerging interest in non-coherent ultrawideband (UWB) communications, particularly as a technology for low-data rate UWB applications, due to its low-complexity and low-power consumption. In this paper, we evaluate the performance of the energy detector (ED) receiver in terms of bit error probability (BEP). We consider an ED receiver based on a conventional square-law detector and binary pulse position modulation (BPPM). We analyze the BEP in a multipath fading channel, both in the absence and presence of narrowband interference (NBI). We consider two cases: 1) single NBI, where the interfering node is located at a fixed distance from the receiver, and 2) multiple NBI, where the interfering nodes are scattered according to a spatial Poisson process. Our framework is simple enough to allow a tractable analysis and provide insights that can be of value in the design of practical UWB systems subject to interference.