Certain CDs not recognised by Guest OS

I'm normally a Parallels Desktop for Mac user but I'm tryng Parallels Workstation out on my work PC to test ZENworks 7. It's provided to me for free under the NHS. Apparently I can use it to help manage my servers and desktops.

I have Parallels Workstation installed on my Windows XPSP2 desktop and have created a Windows Server 2003 SP1 guest machine. It's running beautifully. Just one thing - when I put in one of the many ZENworks CDs, the host machine sees it but the guest won't. If I click on the CD Drive in My Computer, it asks me to insert a disk. If I put in any other CD - Windows, Office, etc it sees it and I can use it without a problem. I've tried disconnecting and reconnecting to the real disk but no effect. Also nominating the CD drive as a shared folder tells me I don't have the correct permissions. I have noticed that when I put some of the disks in, they autoplay on the XP host even though autoplay has been turned off. Yet even when I know the computer can see it, Windows Server 2003 doesn't. I don't have any other virtual machines to try this on.

As far as I can tell the ZENworks disks are just normal CDs.

Thank you.