A recent survey by a security company called Secunia shows that only 5% of computers are fully patched. The other 95% are running insecure software.
It’s important to patch all of your software. This includes the operating system itself (e.g. Windows, Mac OS, Linux), your web browser (e.g. Internet Explorer, Firefox), and all your applications. And of course in an office environment patches should be carried out by IT administrators (complete with backups).