Just a little more testing to eliminate a possible problem or two. All of these except the last are based on a possible problem at the XP machine since it is the only hard wired device from what you've told us.
Next time the problem turns up with the wireless machines try unplugging the Ethernet cable at the router to the XP machine and see if the wireless boxes start working properly. A partially defective cable can cause problems like you've described.
Try a different Ethernet cable and see if that fixes things. If it does not then try leaving things cabled up but power down the XP machine. If that fixes things then the problem is either hardware or software inside the XP box.
As a test boot a Live Linux CD of one kind or another on the XP machine and make sure it is connected to the internet. You can download and burn a CD of Puppy Linux from here »
livecdlist.com/ as the download is around 140MB. Verify it's working with the browser and leave it powered on for a half a day or longer and see if the problem comes back. If not then the problem is most likely in the XP software which might be fixable by using a restore point prior to when the problem started.
Finally was any wireless phone service changed, a new microwave, or any other electronic device added? I have seen removing the battery from cell phone(s) clear up problems with wireless networks, usually just the closest device to the phone is affected but a cellular type booster can wipe out the wireless feed on an entire house.