Do these in the following order:
1. Check that the 3.5mm plug for the speakers is fully inserted. The plug is designed with the signal for the right side in the middle.
http://www.cablechick.com.au/resources/image/trrs-diagram2.jpg
When it's only partly inserted (not all plug/socket combos mate perfectly), the surface for the right audio can make weak enough contact that the socket that its volume is diminished. The left side audio being longer does not have this problem. And if it does, it usually ends up picking up the right audio signal. This is the simplest thing to test so do it first.
2. Try a different set of speakers or headphones. Rather than assume the problem is with the computer, this will allow you to eliminate the speakers themselves as the culprit.
3. If other speakers suffer the same problem, create a Live Linux boot USB or CD. Boot off it and see if the problem still occurs there. If it does, then your computer's audio output socket has failed. If you're lucky, your computer will also have audio out in the front (for you to plug headphones into), and you can simply plug the speakers in there. If not, you may be able to rig up something directly to the motherboard's audio pins, like install a front audio panel.
https://tutorials.ubuntu.com/tutorial/try-ubuntu-before-you-install#0
https://www.amazon.com/2-Port-Microphone-Desktop-Connector-Adapter/dp/B013QSTTUO
4. If the problem doesn't occur with the Linux Live system, only then are you looking at a driver/configuration problem. In addition to what Sedivy has suggested, try looking for any audio-specific apps installed for your motherboard's audio. Sometimes these operate independently of Windows' sound settings, and allow you to control things like sound field and balance (so if your right speaker is a lot closer to your computer than the left, you can diminish its volume so the two sound equal when sitting at your computer).