Create a logoff script with the following registry keys. Doesn't have to be powershell, this code would work for a normal command line batch script. The username is "user" and the password is "user"
reg add "HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon" /v DefaultUsername /t REG_SZ /d user /f
reg add "HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon" /v DefaultPassword /t REG_SZ /d user /f
reg add "HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon" /v AutoAdminLogon /t REG_SZ /d 1 /f
reg add "HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon" /v ForceAutoLogon /t REG_SZ /d 1 /f
If you have a "welcome message", you will still have to click OK to fully log in, though
Here is how you create a logoff script: http://technet.microsoft.com/en-us/magazine/dd630947.aspx