I'd be willing to be its interfearence from a neighbor or your microwave or cordless phone. There's not much that connecting to some other network would do.

However, if you've ever connected to an ad-hoc (computer-to-computer network) then XP will want to default to that connection, even if it doesnt exist. Go into your wifi setup and delete all the prefered or remembered networks exept your legit home network. that may help.

Make a drastic change to the channel...if you are on 6 the go to 11. See if that helps.
What kind of AP is it?