SharePoint’s Enterprise Search Center and Basic Search Center are based on a minimal.master Master Page. This means that you cannot easily integrate a Search Centre into your standard global navigation deployment. To get around this problem you can either open the Search Centre in a new browser window, or implement a fix. My company Deltascheme offer a simple fix for the Basic Search Center. If you choose to install the solution you can create a revised version of the Basic Search Center that integrates with your standard v4.master global navigation.
Follow this link to download the fix: http://www.deltascheme.com/deltascheme-products/basic-search-center-with-navigation-for-sharepoint/