Everything works fine, but when I unplugged the green plug and inserted
the
headphones plug, my whole system froze and I had to reset the PC.

Any idea why?

Could have been a static discharge when you touched the headphone plug to
the connector. That can cause stuff like what you describe to happen. It can
also nuke sensitive electronic components, such as your PC. Always make sure
the computer and all its accessories are plugged into the same GROUNDED wall
socket. Also, make sure to touch the metal chassis before you insert any
plugs, and only plug/unplug plugs that are MEANT to be hot-plugged (ie: not
serial/parallel cables, gameport, VGA etc).

There's no particular reason your computer should reset by plugging in a set
of headphones as the soundcard has no idea wether you have anything plugged
in at all, or what it is; it can't sense such things. If it's not static, it
could be a hardware problem.

Does your box reset EVERY time you plug in your headphones? Do the phones
work; ie, not shorted out? Try a different set of headphones and see if
the same happens.