by reciever, for the Z-5500's, you mean the decoder/control box they ship with? (unless you've actually re-wired the system to use an HT reciever, which is entirely possible)
The TV has RCA stereo output, so the only options for input to the decoder box with the Z-5500's is going to be a stereo input, you could try patching through your PC but that'll require the PC being on to watch TV (not really worth it imo, unless you use the PC for HTPC duties).
A few things to consider:
stereo signals won't become surround sound just because the speakers are surround sound
if you can get a digital/coaxial S/PDIF out that would help a lot more (do you have an STB that produces such an output?)
Generally what I'd suggest is connecting the Z-5500's to the computer as a 5.1 speaker setup, and connecting the TV to the computer as a secondary monitor, and watching DVD's/etc in that manner, which will give you 5.1 sound, and utilization of the size of your TV. The issue you'll run into here is that you can't do much else than DVD's.
The Philips DVP-642 would be another option, if your primary use is DVDs, its about $50 online, and provides a digital optical out.
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