help: wireless lan utilization low

I used to use a 4 port ADSL modem to connect my laptop desktop and internet. both machine have windows sharing opened to lan. When I copy files from one machine to another the speed achieve 80% of 100M lan. that's normal.

Now I added a wireless router D-Link DI-524 with 4 10/100 lan port and 54M wireless access point. My desktop is connected to lan port with cable and laptop using wireless. then I found out that if I copy file between my two machine, I can get a speed of only 1 MB/s (less than 10 Mbit/s), only about 15% of the 54Mb wireless link speed. If I connect my laptop to the wireless router's lan port with cable, I still get 80% of 100M lan speed.

My question is, is it normal to have such small link utilization in wireless network(15% comparing to 80% with cable)? or is it possible that my wireless router or laptop wireless network card have some problem? My dell laptop is using build-in wireless card. my access point is using WPA-PSK (TKIP) with a long key. Could the encryption add overhead? The AP is only 2 meters from laptop, signal level is excellent. but I can also detect several other wireless networks from my neighbours. Could radio interference cause the problem?

please do give me some suggestion. I copy big files quite often and now it takes nearly 10 times long as when I use cable connection

thanks