Very few of us have a reason to use DOS nowadays. (Disk Operating System)
But every once in a while, we are forced to use it for things like bios updates and other unique situations.
"AllBootDisks.com is your number one FREE resource for all Microsoft boot disks.
We offer a boot disk for everything from MS-DOS 3.3 to Windows XP Professional. These disks can be used to setup a new hard drive, scan an existing hard drive for errors, install or re-install Windows, upgrade your PC's BIOS, run DOS utilities and much more." - AllBootDisks.com website
AllBootDisks.com has boot iso images you can then burn to cd.
They have a very useful selection.
To look them over, go to http://www.allbootdisks.com