Sorry, but there is no Windows version of Unison. Unison is a Mac-only program. If you only download files from Usenet, you will probably like Super-NZB, which runs on both Mac and Windows. SuperNZB doesn’t have a search feature like Unison, however, there are quite a lot Usenet search-engine sites available. See the list on the NZB Site Guide page.
If you participate in discussion newsgroups, and upload files, you can use NewzToolz which is free, and also runs on both Mac and Windows.