There's no easy way to use local applications on a PC as the result of an accelerator or a search provider in IE8 but there is a hack-y/obvious way, that I'll describe here. Both accelerators and search providers in IE8 fill in URL templates and navigate to the resulting URL when an accelerator or search provider is executed by the user. These URLs are limited in scheme to http and https but those pages may do anything any other webpage may do. If your local application has an ActiveX control you could use that, or (as I will provide examples for) if the local application has registered for an application protocol you can redirect to that URL. In any case, unfortunately this means that you must put a webpage on the Internet in order to get an accelerator or search provider to use a local application.
For examples of the app protocol case, I've created a callto accelerator that uses whatever application is registered for the callto scheme on your system, and a Windows Search search provider that opens Explorer's search with your search query. The callto accelerator navigates to my redirection page with 'callto:' followed by the selected text in the fragment and the redirection page redirects to that callto URL. In the Windows Search search provider case the same thing happens except the fragment contains 'search-ms:query=' followed by the selected text, which starts Windows Search on your system with the selected text as the query. I've looked into app protocols previously.