Hi Fran
Vista re-indexes files every so often in the background, (Doesn�t seem any logic as to how it works) and if you re-boot or turn off your computer off during the re-indexing then instead of continuing from where it left off on the next start-up, it starts from scratch. (Hence the problem)
It is a known issue with Vista, and as far as I am aware apart from disabling indexing, a fix has not yet been posted.
Sorry to be the bearer of bad news.
You may like to try a 3rd party indexer like Copernic http://www.copernic.com/en/products/desktop-search/home/index.html and forget Vista indexing.
Hope this helps

Bill