Well, the most significant thing is your BDP (Bandwidth-delay Product). This just confirms what you already know - that data transfer on one machine is different than the other (and substantially). Though I am Net+ certified, I wouldn't say I know all the variables that come into play to calculate the BDP. We do know that there is a bottleneck or two in the data transfer though. We also know that is is at your location! Take a look at the patch cables. I assume they are CAT5, or CAT5E Are they the same type?
Now run the optimizer on the slow PC and do some more speed tests (I would not mess with the fast one yet - we need to use it for a guide to changes in the network speed.)