You should be able to see whether or not the rear speaker is recognized by confirming the designated audio output port as the rear speaker. Are you certain that you've set the speaker setting on both the Realtek and Windows audio at 5.1 speaker configuration? 3 out of 5 speaker functioning is most likely caused by incorrect setting.
Could you post the settings? Better yet, the screenshot of the control panel settings.
I *think* I know the problem here: Do the speakers connect via RCA, or are the dedicated PC speakers?
I ask, because RCA uses 1 audio channel per jack, where pc audio (AC97 standard) carries 2 channels per jack. That might explain the hookup problem...If the speakers are AC97, they should be color-coded...
Also, one minor note: AC97 and other audio standards (Dolby/DTS) define the second set of speakers differently. AC97 defines the side speakers as being the second pair, where Dolby/DTS defines the rear speakers as the second pair. This can sometimes mess up an audio setup. A work-around is to set windows to 7.1, and disable either the side or rear speakers as needed.
Worse case, you can try a 7.1 test, just to make sure the speakers are plugged into the right connector.
Eh, Voila! Read the manual. I was trusting the color-coding, and the blue plug for the rear speakers going NOT into the blue plug-in on the Dell fooled me. It goes into the black plug-in, as the manual says. Thanks!