Windows 8.1 has some driver issues on some hardware, particularly with USB 3.0 devices, so if you're using those that may be the issue.
Otherwise, the problem is probably caused by a 3rd party shell extension going wrong inside of Opus.
The Crash, exit or high CPU usage when viewing certain directories guide is a good place to start for tracking down the issue, and the Other Troubleshooting parts of the FAQs may contain other useful guides, depending on what you find or if you are seeing other problems.