@Henrik Pedersen
Under XP you do this through MSGina.dll (or a replacement of it). I don’t know if it is the same for Win Vista/7 or not.

Here is a good MS whitepaper to at least get you started: http://support.microsoft.com/kb/810756

@CC_DKP
Yeah I have looked at MSGina.dll before. It is not the same for win7, I remember reading something about some crazy API shit which is needed. A much easier alternative is to just freeze the screen by blocking mouse and keyboard input but it’s incredibly hard to do “right” as CTRL-ALT-DELETE always seem to work. I have even tried some of the stuff from the more “dark” sides of the internet and some of it seems to block it all in XP but Win7 is actually having to good security against this. LOL.

I know that some facial recogniton programs overlay their own stuff on the logon screen and unlocks from there so it’s definitly possible. Just hard as hell.

You need to hook a custom behaviour into LogonUI. Create a COM object that implements ICredentialProvider and register it with the system. Your component can then tell LogonUI to tell WinLogon to unlock a session.

http://blogs.msdn.com/b/securitytools/archive/2009/07/30/how-to-build-custom-logon-ui-s-in-windows-vista.aspx
http://msdn.microsoft.com/en-us/magazine/cc163489.aspx

FYI

BlueProximity does the same under Ubuntu except it uses Bluetooth and your mobile phone.

@Dosx already considered it… But I personally think the system distance is a little to long (unless you mod it of course)… I already have RFID, magnetic card and soon fingerprint scanner on my door so it would just be overkill to the overkill if I made another unlocking system LOL ! xD

But yeah I own one of these and I can confirm that they send out a unique code for each unit comparing to others sources and that the software that comes with it seems to be able to identify different units, so it would actually make a cheap “powered RFID” system.

for those looking for better software to lock windows, couldn’t you just fire a SendKeys “windows key” + L? that’s a shortcut to lock the workstation…
I don’t know if sendkeys-ing windows hotkeys would even work, but it might be worth a shot if nobody’s tried

I Looooove this kind of stuff. DoSsing such a device with a local transmitter(a 5 W ham radio UHF rig for eg.) is easy. Any user who has his “security wireless key” blocking his computer without any visible reason will immediately “disengage” the security device. And… voilà ! you’ll just have to follow the opened yellow brick road.
By the way, you don’t need a USRP to locate a frequency, a scanner, a grid-dip will do the job for less than a few bucks. The USRP is a great hacking device, but it’s an overkill to do such a simple “evil twin attack”. And it’s not the only SDR on the block (try to google “UHFSDR” or “SSRP”).
Marc