Yes it could easily be a software thing, or maybe a Windows Mixer setting, but usually this is a cabling issue. Your headphones work on the front, great. You're not talking about the jack on the CD ROM, are you? Can you adjust the volume via the speaker icon in the systray, then plug the same headphones into the rear jack, and get sound from both channels?
If you now get stuff on the Right, it's cabling. Maybe you know this already, but except for maybe the mic jack, these should all be 3-conductor stereo jacks, where the 3 conductors are termed "tip, ring, and sleeve".
Usually the user with Left-Channel-Only output problems, has plugged a 2-conductor plug into the jack. This collects the Left Channel sound on the tip fine, but shorts out the ring, which is where the Right Channel sound is. Using proper 3-conductor (stereo) cables and adapters throughout the signal path usually solves the problem.
So if you've still got trouble, check your cables first. If you look at the plug sideways, you should see 2 black bands on it, separating the metal into 3 parts. If you only see one black band, it's a mono cable, which won't work. Sorry if this sounds really elementary, but not everybody reading this message will know audio cabling in detail.
Hope this helps,