There are several sound cards that support Dolby Digital Live (a real-time DD encoder; part of Dolby Home Theater) and/or DTS Interactive (a real-time DTS encoder; part of DTS Connect). The cheapest one (apart from onboard audio of several GIGABYTE mb that supports Dolby Home Theater) is DIAMOND XtremeSound XS71DDL, ~$26.
Right, but that would require using toslink to the receiver from the PC, correct? Currently I have toslink from the TV, and HDMI from the PC to the TV.
Yes, you'd need to run a toslink from the computer to the Receiver; or from the computer to the TV, assuming that 1) the TV can assign the optical in to the HDMI, 2) the TV will then passthru the S/PDIF signal to the receiver without decoding it and downmixing it to stereo; or run analog outs from the computer to the receiver, assuming the receiver has 5.1 analog inputs.
Your only other choice is to buy a receiver that can accept HDMI. The sound card would be cheaper; is there some reason you don't want to run another toslink from the computer to the Receiver?
-- Eric B. Smith The future keeps telling us what the past was about. You make the past mean different things by what you do with the time that comes after.