To support Dolby 5.1, the source has to be recorded in Dolby Digital format. This format is then decoded by the sound processor, and output to the speakers.
Most movies support Dolby 5.1, so you would hear full Dolby 5.1. Games, on the other hand, aren't really a Dolby format, since sounds are generated on the fly, and some games don't support 5.1 at all; it all depends on the source.
The problem with onboard sound cards on a pc is that most older ones only have a single output (headphone) jack, so it can only output stereo. If you have a newer board or a sound card (like a Sound Blaster) with 3 output jacks, or an optical jack, you can output 5.1.
Now, my Logitech speakers have a built in processor, so it will take a stereo signal and output it to the speakers as 5.1. It's not the same quality as if it was originally recorded in 5.1, but it still gives a surround effect. It basically takes the original stereo sound and outputs it at maybe 2/3 volume to the front sattelites, 1/3rd volume to the rear satellites, and full volume to the center and sub...not optimal, but somewhat effective.
You can try setting your reciever to do the same thing. Usually, if you just set it to a movie mode, it will. Otherwise, invest in a Sound Blaster card...you definitely won't be disappointed...