one question to ask is whether the number XP reported is even close to accurate ??
I have seen several of my wireless systems using win 7 and or XP and the router pages all be wildly different..... Windows likes to report a baseline connection speed, but it may not reflect the reality of actual data transfers....
I use the readings from the router's wireless page myself..... Also I test from a site like www.testmy.net and see if I get a large file download speed that is consistent with my other LAN based systems....