In this paper, we investigate the system performance in terms of outage probability and symbol error rate of cognitive relay networks with underlay spectrum access over Nakagami-m frequency selective fading channels. Underlay spectrum access is deployed at the secondary transmitters, i.e., the secondary source and relay, as a means of providing high spectrum utilization efficiency. It is assumed that the whole system operates in frequency selective fading channels which commonly occur in broadband communication networks. In addition, direct communication from the secondary source to destination is present together with relay communication such that the selection combining is applied at the destination. That is, either the direct or relay channel is selected for communication depending on which one provides the best signal-to-noise ratio (SNR). Analytical expressions for crucial performance measures such as outage probability and symbol error rate are formulated. Based on these analytical outcomes, respective system performance is investigated through numerical results for various system parameters and scenarios.