To make an ISO from your CD/DVD, place the media in your drive but do not mount it. If it automounts, unmount it.
dd if=/dev/dvd of=dvd.iso # for dvd
dd if=/dev/cdrom of=cd.iso # for cdrom
dd if=/dev/scd0 of=cd.iso # if cdrom is scsi
To make an ISO from files on your hard drive, create a directory which holds the files you want. Then use the mkisofs command.
mkisofs -o /tmp/cd.iso /tmp/directory/
This results in a file called cd.iso in folder /tmp which contains all the files and directories in /tmp/directory/.
Windows
VaporCD can create a copy of a CD on your hard disk and mount it as if it were the real CD:
http://vaporcd.sourceforge.net/
http://www.granneman.com/techinfo/linux/burningcds/makeanisoimage.htm