Slow connection with 802.11n

I upgraded my desktop PC today, and one of the components I purchased is an ASUS PCE-N15 802.11n wireless network card. This was to replace an old D-Link 802.11g card, because I have been unable to get more than a 20 Mbps connection speed on the desktop. However, I have seen no increase in speed.

My service plan is 50 Mbps, and I consistently get ~45 Mbps on my Macbook in the same room as the PC. I am unable to get a wired Ethernet connection in this room, and I would really like to solve this problem so I can keep the computer where it is instead of relocating it for a better connection speed.

System specs:
ASUS F1A75-V PRO
AMD A6-37670K
AMD Memory PC3-12600 8GB (2x4GB) DDR3
ASUS Radeon EAH6670
Windows 7 Home Premium 64bit

I am going to haul the rig downstairs to test the Ethernet and close-proximity WLAN connections, and will report back once I have. Thanks in advance for any help.

what anti-virus software do you use? or ever security suit? it may well be something over filtering your connection

I'm using AVG Free Edition 2012.

It tests ~45 Mbps on both Ethernet and WLAN when it is in the same room as the router, so it seems that when it is in my room it is picking up some interference that my Macbook is not when in the same place. Any ideas?

Wow. Back upstairs, I repositioned the antennae and it now tests at ~40 Mbps. My neighbors must have something on the other side of that wall.

It's jumping between 15-30 Mbps now, so it seems like I'm not going to get decent speeds unless I have the tower right in the middle of the room, which would make for a very expensive morning when I crawl out of bed and trip over it. The interference is outside of my ability to diagnose/correct it, so this thread can't really be of use to anybody now. A mod can lock it.

This sound weird, I have the exact same wireless card as you have, but what you have is most likely a erm not sure what its called in English but a direct translation from my language would be a "shadow area" and I had it at the old place I lived in, I could only get good/decent wireless connection in a certain area of my room, What you need is probably a access point to increase your router range. Can be done with any wireless router what I know of, and there is a guide on this forum explaining how to set it up.