We present spherically symmetric interacting-winds model of planetary nebulae aimed at the study of the hot bubble in their structure. A similarity solution for the hot-bubble structure is derived in the case of a time-dependent wind of the central star and taking into account the effects of thermal conductivity. This solution is used to model the X-ray, EUV and infrared-coronal line emission (lambda > 1 mu) from planetary nebulae. These results can be used to check the consistency of the interacting-winds model by a comparison with space and ground-based observations with high spatial resolution.