Is there a secure desktop operating system I can use today?

Context: Qubes OS is a desktop operating system based around the Xen hypervisor that allows grouping programs into a number of isolated sandboxes to provide security. Windows for programs running within these sandboxes can be color coded for easy recognition. The security domains are configurable, they can be transient, and their network connection can be routed through special virtual machines. The operating system provides secure mechanisms for copy and paste and for copying files between the security domains.

Answer: Use Qubes OS! It is fairly complete and provide good isolation for most of your applications.