My SD card isn't showing up on my computer?

I have an Acer Aspire 5610 and a Sandisk 2 gig memory card and a Polaroid 1 gig memory card. When I put them into the SD slot, they don't show up anywhere in my computer and every website I've tried hasn't worked. When I take the cards out, my computer makes the noise it does when I remove things. So it's there, but it's not showing up anywhere?