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I had the same problem with a Dell when I unloaded Win 7 32-bit and installed Win 7 64-bit and discovered the four card readers didn't show in Windows Explorer, but did in Device Manager using Windows generic drivers
I later bought a Kodak card reader for my wife's laptop and tried the Kodak software on the Dell and lo and behold, all four card readers showed up in Windows Explorer.
So, there you have it, Dell don't supply card reader drivers for the 64-bit version, but a third-party provider may do.
As a matter of interest, have you tried a card to see if Dell's card readers work?