To have your hardware to work with the OS (e.g. Windows 7), a "driver" is required.
Windows 7 has most of the common, newer types of hardware drivers, so your SD card should show up under "Computer" However, some brand name PCs (Dell notebook?) need some drivers Not included in Windows 7.Take a look at their support web site to download the driver. Also, 32-bit driver will not work under 64-bit Windows 7.