As you probably have found out, its very difficult to find a computer with XP installed. Usually they only offer new ones with Vista or now System 7.
I tried in vain to buy one with XP but finally bought a new laptop with Vista and I now I think Vista has got an undeserved bad rap. In fact I was so impressed by it that I also installed Vista on my wife's laptop computer which use to have XP and ran very, very hot but with Vista it seems to run a lot cooler. So it asks you to verify when a program tries to run, big deal, it just prevents unwanted intruders getting in and destroying your PC. Granted you will need more memory but now days 2Gig is the the minimum for new PCs anyway.
Or you could go with a new PC and put on Linux which a lot of people on this thread swear by.
There is another thread on here talking about the long-distance wifi:
Long-Distance WiFi Device
I just got the one recommended by them the unit is the Alfa 500mW USB wireless-G Wifi adapter,w\threaded antenna jack.$47.95 US.The antenna is what makes it work,get the Engenius 2408 omni antenna $60. Some say it works five miles away. I haven't tried it offshore yet but at my dock when I use it the signal is much stronger and I pick up many more wifi's.