Very important is signal level on your computer, good tester is inSSIDer, you can search in internet this software.
Next, what is your Wireless Settings on routera, give us screenshot from this page.

I know inSSIDer... my signal varies sometimes but it is usually between -45 (if i'm in my office) and 15 if i'm in the same room as router.... but speed doesn't varies so much...unfortunately.

here are the settings:

I can't see anything because resolution is too small. Please correct screenshot.

I'm surprised because on my computer resolution is ok and perfectly readable.
the file is 70 KB sized on my computer but every time after upload on this forum, it's only 18 KB... so problem is with the upload method of this forum.

please have a look at https://picasaweb.google.com/117641323412228734768/February162013#5845486082919314578 and inform me if you can't read it.

thx

Try make mode: b,g,n mixed, next click save and reboot your router. What is your wireless card model?

I already set it to mixed b,g,n and i had the same result.
i have tested several wifi cards and USB antennas.
with 36dB USB antenna i have a better speed (almost 11 MB/s) but nothing fast.

not i changed my MB and i have a P8-Z77 V from ASUS. Wifi adapter is a "b/g/n" bands compatible.
here is the model ( http://www.asus.com/Motherboard/P8Z77V/)

My last idea please set system in safe mode windows with networking and then you can make tests. Can you see then difference?

ok, i tested it in safe mode with networking under win 7 and unfortunately it is worse...only 2 to 4 MB/s as speed when i transfer files to my NAS.
i really wonder what is bad with wifi... i had a better speed (not 200MB/s also) under Fedora 18 / ubuntu 12.10.

if i use directly cable RJ45, i can save file to my NAS up to 22 MB/s which is still not good enough, but it is still better than thos 4 MB/s in wifi...
so i see several issue here:
1. wifi has some issue with my router...settings on router or on all wifi cards/USB antenna
2. transfer settings from router to NAS is bad also, as NAS is a GB LAN so 22 MB is far from GB LAN.

I think that is something wrong with your system or wireless card drivers

I did some more tests using jPerf from computer to computer.
tests were 20s long each, average speed is around 24.3 Mbits/s when both computers are in safe mode with networking (and without running any AV or firewall).
if both computers are in normal mode JPerfs shows a 33 to 35 Mbits/s speed...which is still far from N band speed

so problem is in card driver/settings OR in wifi router tp-wr1043nd :-(