You have to do things a little differently so people with Windows can read an iPhoto CD from your iMac. Rather than burn a CD from within iPhoto, you need to use what’s called a burn folder from the Finder, and then your CD will be usable for people with Windows.
It’s easy to do, it’s just an extra few steps to take for Windows and photo printer compatibility.
Once you have a blank CD in your iMac, you can export your album or albums from iPhoto, select the file type (usually jpg) and the quality, and export to a burn folder, and then make the CD. There are complete instructions at Apple: iPhoto: How to burn a CD or DVD with pictures for a Windows-based computer.