Atheros Wifi adapter doesn't work with "n" router

Hello
I have an HP pavillion desktop (s5360f) with Atheros 802.11 a/b/g/n Dualband Wireless Module and D-Link router DIR-615. My numerous Laptops and iPhons / iPads work fine, but this desktop which is further away from the router (in another room, about 20 meters) only shows 1-2 bars of wifi. First I bought a big additional D-Link antenna (7Ghz) but it didn't really help - sometimes connction shows 3 bars. I was told this antenna should help a lot. Then I checked with my router settings and saw that the HP uses "g" connection when everything else uses "n". I even set it to only use "n" but still it shows the HP using "g". I updated the driver (Atheros 9.2.0.113 for AR5B91), but it didn't help. I have "n" enabled in Atheros in Device manager.
Now I'm lost and can't think of anything else that can be a problem and / or how to fix it... I don't have any thick walls between the router and the HP, my iPhone seems to work in this room just fine...
Please help!
THANK YOU!

Thank you for your fast response!
Yes, when it shows 1-2 bars surfing the web is normally a little slow on it's own but when I'm downloading something at the same time - almost impossible. I never have this with other computers. I actually just checked with my laptop at the same location, same OS, wifi settings but Atheros AR9285 - has great connection, uses "n".
I'm not sure how to check for speed... when I open Network and Sharing Center and look at wifi properties it shows 130 Mbps (yeah, right!), Vistumbler shows 25-40% signal. Is that what you were asking about?