It seems like syncs from PC-to-iPhone, and iPhone-to-PC are unique, and one-directional.
This is why you can't take the photos sync'd with your g/f's computer, and then re-sync them to yours. I don't believe the iPhone supports this.
As has been stated here, it should be possible if you are willing to jailbreak your phone (and therefore, technically, void your warranty). Can you not find a simpler method to sync photos between 2 PC's?
"When fascism comes to America, it will be wrapped in a flag and carrying a cross."