Gibson Research Corporation writes on Ultra High Security Password Generator:

Since there is no "output from the previous encryption" to be used during the encryption of the first block, the switch shown in the diagram above is used to supply a 128-bit "Initialization Vector" (which is just 128-bits of secret random data) for the XOR-mixing of the first counter value. Thus, the first encryption is performed on a mixture of the 128-bit counter and the "Initialization Vector" value, and subsequent encryptions are performed on the mixture of the incrementing counter and the previous encrypted result.

 The result of the combination of the 256-bit Rijndael/AES secret key, the unknowable (therefore secret) present value of the 128-bit monotonically incrementing counter, and the 128-bit secret Initialization Vector (IV) is 512-bits of secret data providing extremely high security for the generation of this page's "perfect passwords". No one is going to figure out what passwords you have just received.

Make sure to write the generated password down.