A few things could be slowing down your network...
a) interference - Wireless operates at 2.4GHz which is a nice band to go thru walls and other hard surfaces, this is exactly why some portable phones use it. (See the problem?) There was a problem with 900MHz CPUs and 900MHz phones (Although I never personally experienced such, so I can't vouch for that one, just a thought) could be the same if your PC is running at 2.4GHz

Too many devices on the network - If your using wireless do realize that if your using 802.11b your only going 11mbit in the best conditions, this translates to 1.375Mbytes/sec (give or take) less network overhead and attenuation, SPLIT among all your wireless devices. (54mbits if your using Wireless G)
c) Your PC's TCP/IP stack is screwed
d) Your PC's TCP/IP stack isn't optomized for ethernet (MTU/MSS/RWIN etc)
There could be more, but I'm kinda tired at the moment.