Customizing Error Message During Registration

[induslady]

Hello,

In the register page, in the field - "Username"
When one enters an already existing user name or special characters / space which is not allowed in the user name, it gives out error message -

$vbphrase[usernametaken] - That username is already in use or does not meet the administrator's standards. If you are {1} and you have forgotten your password, click <a href="login.php?{2}do=lostpw">here</a>.

But my requirement is that this error message should be customized such that -

1)For an already existing user name the error should be -
That username is already in use. If you are {1} and you have forgotten your password, click <a href="login.php?{2}do=lostpw">here</a>.

2)For special characters and space the error should be -
That username does not meet the administrator's standards.

Let me know how to do this? Thanks in advance.