The fact that you were able to get Windows 2008 installed on VMs tells me the physical hardware is just fine.
I think it is one of two things:
1) I think whatever virtual storage controller you are using in the VMs you created for Windows XP and Windows 7 do not have drivers included in the base install for those versions of Windows. Thus, the Windows install is unable to see the storage contoller. Did you use the recommended controller for Windows 7 and Windows XP? Did you correctly set the version of the operating system when you defined the VM?
In this case, you need to hit F6 during the text-based part of setup for Windows XP, and install the VMware drivers for the scsi controller. If you edit the floppy drive in the VM's properties, you can link the drive to an image located in a datastore. Select this option, navigate to the folder /vmimages, and select the vmscsi image.
For Windows 7, I don't remember where you add drivers during the install, but there should be an option to do the same.
2) You didn't add a virtual hard disk to the VM.