I managed to get my card slots to work for a while by reinstalling the drivers. It seemed that a driver from Pinnacle for the USB HD TV tuner driver was driving the USB device related to the card slots. I reinstalled the Windows drivers and all was well... for a while.
I placed an XD card in the SD/XD slot and it read fine.
Today after a reboot, about half the day the drive letters were showing. I went out and took pictures. When I put the card in the card slots the drive letters had gone.
I'm not sure that the Pinnacle USB TV is the only culprit. What might happen is that Windows Media software goes and looks for a TV guide only to mess up the system.
The other thought I had is that maybe I have too many hard drives in my system - but these are USB ports so that shouldn't matter - should it? I have 3 internal drives partitioned and an external USB drive, plus the card slots, plus my Epson R300 printer seems to want to take a drive letter - which I can't read.
Once again when the drive letters disappeared I also had an HP iPAQ hx2415 and an Apple iPhone plugged into USB ports - as well as various other printers, in all about 10 USB devices linked to powered hubs.