Menu item gets displayed on multiple lines

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...
  i used orientation=horizontal ppt...but still it is not rectified
   for eg... my menu items are     "Register"   "Database"
   when the alert get displayed my menu tab is displayed as
    "Register"
    "Database"


                                     and the alertbox is displayed here(center of the page)!!!    ...