WiFi - slow download, fast upload. Affects only one device

WiFi - slow download, fast upload. Affects only one device

My desktop computer is getting terrible download speeds, while other
devices work just fine on the same network. Here are speedtest.net results
from my devices, Mbps:
Nexus 4: 24.9 down / 26.8 up
MacBook: 31.3 down / 28.5 up
Desktop: 6.6 down / 28.2 up
All three devices connect to the same network (also tested in the same
physical location), hitting the same remote server. Results stay fairly
consistent between runs.
The desktop PC uses a TP Link TL-WN881ND PCI-e wifi card and runs Windows
8 (identifies as 'Qualcomm Atheros AR9287 Wireless Network Adapter'),
default drivers, no additional wifi-related software installed.
My router is a TP Link TL-WR841N, default firmware. I have tried disabling
WMM, changing router mode / channel width - none of those improved
download speeds on the desktop.
Also, tried using my phone as a wifi hotspot and got the following results:
MacBook: 10.0 down / 1.6 up
Desktop: 9.0 down / 1.6 up
Much closer this time - problems with the specific wifi card / router
combination?