Try this?:
In Windows, double-click on the volume speaker in the system tray area. Open the first menu on the top-right (where the File menu normally is) and make sure "Advanced Options" has a check mark beside it. (If it doesn't, click it.) Now, under your main volume control in the Volume Control Panel, there should be and "Advanced" button under the slider. Click it and see if you can adjust your Treble/Bass settings. (If they're grey, your soundcard/drivers don't support this.)
Also, if your soundcard came with its own software to adjust settings, open it up and see if you can disable fancy effects, or change some EQ levels to make things better.
If all else fails:
1) try replacing your sound card audio wire (try unplugging / replugging it first; sometimes they're loose inside where they're supposed to be tight, this is usually from having the wire stretched to its limit for a long time)
2) get new speakers. I like Altec Lansing. I have a 12-year old pair that will still blast the socks off of most of the cheap sets that come with new computers.