I'm cross-posting with https://forum.nfcring.com/topic/683/nfc-reader-writer-for-windows/20 as this thread is about the same topic.
I'm Maxime from ISLOG and we just put John's advices in practice (he tried our RFID Logon solution months ago and wasn't completely pleased with it previous state).
ISLOG Logon is now available freely for personal use under ISLOG Logon NFC Community package.
You can download it here :
64-bit: http://download.islog.com/islog/4.0/NG/ISLOG Logon NFC Community-4.7.0708-x64.exe
32-bit: http://download.islog.com/islog/4.0/NG/ISLOG Logon NFC Community-4.7.0708-x86.exe
This is a first version for feedback, please be gentle :). No data encryption for now but it will come.
It unfortunately cannot be open-sourced because we have to re-architecture a lot the current codebase to do so (some modules are shared with our commercial product) which is too much investment for our small team right now. So except there is something like thousands of requests we will not do it, sorry. Small comfort, part of it is already open source like the RFID / NFC middleware https://github.com/islog/liblogicalaccess
EDIT: see https://forum.nfcring.com/topic/782/islog-logon-windows-nfc-login