Now, wireless technology ain't that fast man. 54Mbits/s (bits, NOT bytes man) is the fastest, most wireless connections are weak, thus they limit themselves to 11Mbits/s. This is what's happening to you probably. How far is the wireless adapter from the wireless router?
anywho, 54Mbits/s (your wireless max) is 6.75MBytes/s max. Remember, 8 bits inna byte. So a 100Mbit/s (average wired connection) can theoretically go 12.5MBytes/s. So yeah, lol, impossible for a wireless-g router and adapter to go 11MBytes/s unless by MB you meant bits, but usually you write the bits out, and for bytes you just put a B. Kinda threw me off, so just wanna clarify this stuff with ya.
Wait a second.. 802.11b is needed to reach full speed for your wireless-g adapter/router stuff I think, in order to do that, you need to have Windows XP SP2 as it adds support for IEEE 802.11b standard. There ya go, try upgrading to WinXP SP2, should help.