How can fix downloading photos onto my Pentax Optio 230 camera?

I have routinely downloaded photographs from my Pentax Optio 230 onto my Windows Vista computer for more than a year. Suddenly I am unable to do so and a message is displayed saying Windows needs to install driver software. What has happened to the software I originally installed? Unfortunately I seem to have mislaid the CD that came with the camera. What is now the best solution apart from buying more software?