Hello all,
Richard here - been lurking for a while, first post.
Seb,
If you only have a few users to deal with then I concur with the Rev,
Google Desktop is a great solution; it's simple and it will meet your
users needs. There are alternatives, I actually use one called
Copernic Desktop myself (although mostly for mail) - it works for me.
If you are looking for something for more than a handful of users,
then I'd seriously consider spending $$ on a Google mini appliance.
They are about $6k for two years, which includes maintenance and
support (and you get to keep the box at the end).