OK, you've got SPDIF outputs on the TV, and analog stereo inputs on the receiver. Do you have analog outputs on the computer? (headphones, line out jack...)
If so, you've got the option:
Follow sblair's answer, and hook the computer up to the receiver. You may need to tweak your software to send audio out the stereo jacks instead of the HDMI.
Get an SPDIF-to-stereo converter box to sit between the TV and the receiver.
Converting the SPDIF digital audio into analog stereo isn't a job for a simple adapter, you need a device like this converter[*]:
[*] For illustrative purposes only. Not an endorsement of this product or the store providing the URL. This particular model only handles PCM, not DTS/bitstream digital signals.
Does the receiver have an HDMI output paired with its HDMI input? If so, you could daisychain from computer -> receiver -> TV and stick with HDMI all the way.
What model is your receiver? You mention having an RCA input on it; is it a digital input (single orange RCA) or analog stereo input (two RCA jacks, red & white)? The device Bill Rodman's answer links to will only convert digital optical to digital coax, so it won't be useful for converting to analog inputs.