Adding custom search providers to IE7 and IE8 was a bit convoluted, involving going to the Microsoft site to create the required XML based on a template URL. I could never work out why they didn’t build it into the browser, but it did kind-of work.
However, the MS site no longer offers this. A web search gives plenty of blogs telling you how to do it using the Microsoft site, but the option described is no longer there.
Fortunately, Eric Lawrence has put the functionality on his EnhanceIE.com site here:
www.enhanceie.com/ie/SearchBuilder.asp
Thanks Eric!