http://sharepointroot.com/2011/05/25/replace-sharepoint-delegate-control-smallsearchinputbox/
First step is to register the search box assembly to the master page files.
<%@ Register Tagprefix="SharePointWebControls" Namespace="Microsoft.SharePoint.Portal.WebControls" Assembly="Microsoft.Office.Server.Search, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
Then Replace SharePoint controls search box which is the syntax like this
With this one
The search box property can be found on MSDN reference link here
Hope this helps,
PJ