What is this "adapter"? Have you tried connecting directly into the modem with a short Ethernet cable and resetting the modem/restarting the pc?
I've been having a similar issue with one PC in my office in our newly built home. My older PC would work, albeit slower than it should. Reinstalled drivers, tried static IPs, even reinstalled windows, just in case. Finally, I plugged directly into my router - voila. Bad cable run. @%$#%
Hopefully I can just re-punchdown the terminals and its not the actual cable. Checked the rest of the house and have another bad run. This one caused me to buy a new switch in this location before I realized it was the NEW cable run.