I did find a couple of other things to check if your memory card slot letters - like drive letters - disappear.
1. I do believe that some services were turned off that should have been on - I'm not sure exactly what services these are - but some are regarding cards, others Windows Media.
2. I rebooted the machine and the drive letters corresponding to the memory slots on my HP a6152n computer were showing. After I'd plugged my PDA (Windows Mobile 5 iPAQ) and iPhone into their USB ports the drive letters disappeared.
3. I had another problem on one occasion when the drive letters appeared - when I put the card in the slot - despite the green light illuminating - it was unrecognized and the computer told me to put the card in - but it was there already. Maybe I needed patience.
4. I then turned on services that I thought might be needed - unplugged my PDA and iPhone and rebooted - the drive letters reappeared. I put the memory card in - it was read - Windows Media software said there was nothing on it. Horrors - the finder, shell, explorer - whatever you call the basic Vista OS view of the files showed my pictures on the card. I copied them to the computer - finally...
I believe the computer sometimes runs out of resources or has conflicts on the USB ports - maybe it just takes time for all the software to load.
Hope this helps anyone who has similar problems.