A Windows PC will count as a separate device, and its Kindle ebook files will be locked to that particular PC. You can associate a new device (e.g. a Windows PC) with existing ebooks.
The limit is on simultaneous devices, but Amazon isn't setup to automatically handle unregistering a device from ebooks. Also some ebooks have a lower limit, which is listed on the ebook's web page on the Kindle store. See
Amazon DRM limiting number of times you can download a book.