Is there an AirDrop client for Microsoft Windows?
No, I don't believe you can. Apple advertise AirDrop as: "AirDrop, a remarkably simple way to copy files wirelessly from one Mac to another with no setup;"
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Key part of that sentence being "from one Mac to another" - Second bullet point: http://www.apple.com/uk/pr/library/2011/02/24Apple-Releases-Developer-Preview-of-Mac-OS-X-Lion.html. It's highly unlikely Apple would open this feature up to Windows or Linux because it's a selling point for them in new Macs running Lion.