first we will start with the basics. I want you to check all the windows properties to make sure everything is correct. im working off win2k so little things will be different but not to a large degree.
first, goto your volume control>click file>make sure advanced controls is checked> go back to volume control and click the advanced button under "main". this will bring up different things for different soundcards. This is also usually where the bass/treble adjustments may be.
next, goto start button>control panel>sounds and multimedia>audio tab>click advanced in the playback area> make sure settings are correct for your system. If your unsure then just select desktop speakers. (i think your speakers will work fine at that setting because they are driven by one jack on the photos from logitech.com) the hardware acceleration and sample rate should be either best or one step below. if you find this creates problems then keep adjusting down until you like it.)
make sure that there is not a software client installed from the soundcard manufacturer that has any adjustment settings like stereo, reverb, bass/treble, etc. if there is then dig through that for bass/treble settings.
I did not find any adjustments to be made at the speaker so it should be a software/setting issue.
now if after all this your bass is still off the charts, then i would look to a speaker malfunction, most likely the electronics inside the speaker itself.
As for getting your full surround going understand that depending on what you use will determine how its setup. you need to find out what you need. If your stereo speakers are self powered then just plug them into the correct jacks out of the soundcard. If you have to run an amplifier(maybe a stereo?) then just plug into the back of the soundcard into the channels you want the speakers to live at(rear, center, etc..), then plug into the "aux left and right input" on the stereo. be aware that soundcard outputs are line level so make sure you dont plug into a turntable jack. Then just plug in the speakers making sure to keep the speaker wires correct.(+ to +, - to -)
you may end up needing cables with differant types of connectors on them, you can goto an electronics store and get these. if they dont have premade then just make em yourself. (its not difficult, just use patience.) i can see you needing a pair of 1/8" mini mono to rca mono adapters and maybe some speaker cable. dont get to big or to small of cable, stick to 14 gauge for low wattage and no bigger then 10 gauge for high wattage. By high wattage i mean 600 to 10,000 watts. low wattage i mean under 100 watts. I usally go for 12 gauge as it works out for most applications.
there are times whe you want 8 gauge but not for what your doing.
hope this helps. (sorry for the lack of capitols, i'm trying to cut down lol.)