Have you updated all firmware and drivers?
Remember that Microsoft won't do this for you.
I have a wireless b linksys router w/ two wireless cards. One on the notebbok and one on the 2nd desktop. I have a total of 3 computers. 2 of them are wireless. With the noteboock and the wireless desktop my connection is constan but will range speed. I can have both wireless computers sitting 6 feet from the router and will get real crappy speeds on the internet. Is it possible that I have someone outside of my two computers accessing my wireless connection. And if so what can I do to identify and rectify the situation. Thank you for any advice you can give on this topic.