Great for media centre or kids PC's but not for business PC's or sensitive data:
Microsoft Autologon enables you to easily configure Windows' built-in autologon mechanism. Instead of waiting for a user to enter their name and password, Windows uses the credentials you enter with Autologon, which are encrypted in the Registry, to log on the specified user automatically.
Autologon is easy enough to use. Just run autologon.exe, fill in the dialog, and hit Enable. To turn off auto-logon, hit Disable. Also, if the shift key is held down before the system performs an autologon, the autologon will be disabled for that logon.
(it works with windows 8 as well)