[markzi]

When I plug my headphones into my desktop, sound comes out of one ear. The headphones are Sennheiser HD 598.
When I plug other headphones, it works great.
When I plug my Sennheisers into another PC or an iphone, it works great.
The other headphones have cables going into both ears, the senns have one cable going into the left (that's where the sound is going too)
How do I fix this?0

 

[NA Blur]

To me it sounds like the jack on the HD-598 is not fully seating properly into the PC with the problems.  The reason you hear one channel and the fact is is not very loud gives rise to this problem.  Without a ground and only the tip and at most one ring connected you are only going to hear one channel and if the ground is not connected it will sound really faint.
 
Try an extender or some other adapter and see if that clears things up.