debian is a OS (linux/gnu) it is generally not prone to viruses and is generally a fairly secure system but it's security can still be considerably improved, debian hardened is an example of this.
Other simple improvements can also be made, for example not responding to ping requests with makes your machine IP address more invisible to crackers on the internet.
It is available for free download from www.debian.org and worldwide mirrors. Or you can buy CD or DVD install disks for tens of dollars. It currently has 8710 packages available, most of which are open source and all are free to download at no cost. It is a top notch os with full graphical desktop support (gnome or kde).
Or if you want to try debian without effecting your current system try knoppix (knoppix.org click to english version) which is a bootable CD that runs a fully fledged debian based os from the CD.
