In general this code is bad practice...
mysql_query("INSERT INTO table (Password) VALUES ('".md5('$_POST['Password']')."')");
... eventhough it works fine here without having the risk of sql injection as the unverified userinput
($_POST['Password']) is hashed before inserted.
But the selection is missing in this insert statement, in this case the primary key to identify the user who want's to set the pw.
If you are interested in verifying the pw strength (nr of chars, occurence of upper/lowercase letters, spechial chars, numbers...) on serverside I'd transport the pw cleartext from client to server.
I'd also salt the hash instead of using the plain hash, to defend the pw in the database against rainbow table attacks.
Keep in mind that hashing != encrypting, as a hash can not be "unhashed" (but it might be found in rainbow tables if not salted).