In reply to haydn: Ended up downloading a whole load of programs, and I'm still evaluating them
Some aren't good at handling IPTC tags (title, keywords, author, etc) in JPEGs so I've binned them.
I'm using Exifer to tag all my photos, but it doesn't have a keyword searching interface.
I quite like ACDSee and ThumbsPlus, but I've got about a dozen other programs to install and try (!)