menu tab gets displaced when alertbox gets displayed

menu tab gets displaced when alertbox gets displayed

ClientScript.RegisterStartupScript(this.GetType(), "myalert", "alert('" +
"Register already number exists !!!" + "');", true);
/* above is my exception handling code used to display an error message.
when it get displayed, my menu items get displayed in multiple lines...
for eg... my menu items are "HOME" "CONTACT" when the alert get displayed
my menu tab is displayed as "HOME" //"\n" i mean next item is printed in
newline "CONTACT"
display the alertbox here!!!
how can i correct it ? */