It's called the "Century 21 Sting", a short string piece that was used to preface all of Gerry Anderson's productions in the late 60's, like Captain Scarlet, Thunderbird 6, etc. etc. I have only used it out of nostalgia - to me as a kid, that little clip of music always signalled the start of something exciting

"Anything can happen in the next two days!!"

- and it usually does