Like you, my wife and I take lots of photos, thousands at a time, since we are involved in helping to generate an on-line database of Commonwealth wargraves.
With regards to Baltboy's advice - the first part is worth a try, I use a card reader myself. For the second part of his advice - re-formatting the cards - this is a counsel of desperation as it will wipe ALL the photos on the formatted card.
What size of card are you using ? Certainly XP cannot natively read 4 Gig cards, but Win 7 should be able to read large capacity cards straight out of the box. I don't know about Vista, I skipped that one.
One thought did occur to me. A lot of SD cards are 'lockable', there is a little slide sitch on the edge of the case. If this is in the 'locked ' position it might explain your problem.
Another approach might be to see if the computer you are using has a built-in card slot. My wife's Fujitsu Amilo, which is the laptop we take on shooting trips, has such a slot and can read high capacity SD cards with no problem. These slots, connected directly to the mother board, seem to work better than anything else.
Best of luck with recovering these photos, I know what it is like to lose images. I had 400 just evaporate while I was transferring them to the computer last April. Luckily I discovered the loss before we left the area and was able to go back and re-shoot.