Okay, I've been trying to sort out how to explain this to you, so bear with me.
Connect the computers that DON'T have internet to your wrt310n.
Plug the desktop(the one with the um175) into the wrt310n's WAN/Internet connection.
Make sure that in Device Manager that the desktop with the um175 has two working ethernet cards.
Then run the ICS(Internet Connection Sharing) Wizard.
Let me know if part of this seems vague, or you have any troubles with ICS.
If it works flawlessly for you, please let us know, that way I can stop looking for solutions.