Depends on the manufacturer's design of the PC. Is the cable shielded and how it is designed to block RFI/EMI from entering or exiting the equipment it is connected to.
Since a power cord is composed of a hot and neutral wire that the component sits between, a change in the length of the cord will increase the size of the "buffer" around the component. In the specific case of Shunyata Research -- we use patented noise-isolating geometries, shielding and a patented compound that absorbs EMI in some power cord models. Increasing the length of the cable, increases the noise isolation, or coupling effect to the ZrCa-2000 compounds, therefore increasing the performance of the cable..
In general, Shunyata Research does not recommend a power cord that is shorter than 3 feet or 1 meter in length for performance ease of use and, or resale reasons. Of course, subtle degrees of audio performance are not the only consideration when putting together an audio system. Aesthetics are also important especially when the system is located in a beautiful home. I just point out the performance differences so that people can make an informed decision when determining the optimum length for their cables.