Wireless N Network but slow speeds

Hey guys I'm trying to set up my old computer as an HTPC because I found out I can play HD movies on it thanks to Avivo. In order to do this i bought a 802.11n usb adapter so that I can transfer my media onto the HTPC. However the max speed that the adapter will connect to the router is 54mbps. And when I try to copy files it goes at about 2.3MB/s and when I try doing online speedtest it shows about 70% of what I get when i hook up directly to the router. I did all of these tests with my old PC sitting directly next to the router.

In the router settings page it says that my adapter is using 802.11g mode.

Old PC = Windows xp / ati 2600xt
Router = D-link Wireless N (DIR - 625)
USB Adapter = Comstar Wireless N (uses REALTEK RTL8192U Wireless LAN Driver and Utility if that helps in anyway)

Anyone know why this could be happening?

EDIT: The adapter signal strength shows 'very good' when its at 54mbps