Windows Vista allows you to add custom search engine to start menu instant search box

You can add various search engines like Wikipedia, Google, Yahoo,Ebay, Amazon,Youtube and more.

To add Search Wikipedia or Google or Yahoo at Start Menu Instant Search Box, follow below steps:

• Click on Start button, then type gpedit.msc in the search bar, and press enter .
• Press OK on security dialog displayed.
• Navigate to “User Configuration > Administrative Templates > Windows Components > Instant Search > Custom Instant Search Internet search provider”
  click on Properties to open the properties window. 
  
• Go to Setting tab, select Enable .
• At “the string or DLL resource” text box, type Google Search .
• At “the URL to use when invoking the custom internet search” text box,Fill one of the desired search engine.
  For Wikipedia: http://en.wikipedia.org/wiki/%w
  For Google: http://www.google.com/search?q=%w
  For Yahoo: http://search.yahoo.com/search?p=%w
  For Ebay: http://search.ebay.com/search/search.dll?satitle=%+&category0=
  For Amazon: http://www.amazon.com/s/ref=nb_ss_gw/103-7114533-6465427?url=search-alias%3Daps&field-keywords=%+
  For Del.icio.us: http://del.icio.us/search/?fr=del_icio_us&p=%+&type=all
  For Digg: http://www.digg.com/search?section=news&s=%+
  For Youtube: http://www.youtube.com/results?search_query=%+
  For MySpace: http://sads.myspace.com//Modules/Search/Pages/Search.aspx?fuseaction=advancedFind.results&searchtarget=tms&searchtype=myspace&t=tms&get=1&websearch=1&searchBoxID=HeaderWebResults&searchString=%+&q=%+
  For LifeHacker: http://lifehacker.com/search/%+/bydate/

    

• Click OK .
• To enforce new setting click on start button
• And type gpupdate/force .in search box and hit enter key

You can find more and use any search engine. I listed the main ones only.