It’s very useful security measure to force users to use a certain minimum password length, prevent blank passwords.

HCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\Network

And to:
HLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\Network

The first is for the user currently logged in, and the second is the system-wide setting.

Set the value for MinPwdLen to the minimum number of characters that the user must use for his password. Obviously, zero-length passwords will be disallowed if you enter any value greater than zero. This does not apply to existing passwords-only to new or changed ones.