Many years ago, Microsoft allowed users to logon to Windows clients utilized a set of cached credentials. These were credentials from a previous authentication with a domain controller. This allowed for two things.
1) The user could log on to a mobile computer without contacting a domain controller and,
2) The user could logon to the client before the network connection initialized. This gave the appearance of Windows booting faster. It is also why the hard drive is still going strong on the I/O operations after you log in.
By default, Windows 7 stores the last 10 logged on users credentials and Windows Server 2008 stores the previous 25. You can change these values to better suite your organizations needs.
Type Regedit and press Enter.
Browse to HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\Current Version\Winlogon\
Change the value of CachedLogonCount to any value between 0 and 50.
Giving the value of zero will disable cached credentials. Any value above 50 will be treated as 50.