not really clear what the issue is (or at least I am not understanding it)... but you may find luck changing the code behind to this...
Code:
protected void RegisterCheckBox_CheckedChanged(object sender, EventArgs e)
{
if(!IsPostBack)
{
CreateUserButton.Visible = RegisterCheckBox.Checked;
CreateUserButtonDisable.Visible = !RegisterCheckBox.Checked;
CreateUserButton.Enabled = CreateUserButton.Visible;
CreateUserButtonDisable.Enabled = false;
}
}