It's downright common these days to download programs as ISO images, but XP doesn't have a native viewer for them. The Virtual CD-ROM Control Panel is an easy way to create a virtual drive letter, point it at the ISO, and then it 'mounts' the drive so that it looks like you have the files on CD. As far as I know it doesn't support DVD images. It's free and easy to use.
It also doesn't work on Vista.
