Cannot use ports between systems

My LAN consists of 3 Win 7 systems (#s 1-3) + 1 Win XP Pro system (#4). The Windows shared folders work fine between all 4 systems. Until a week ago, I was able to ping the systems and also access the Win 7 systems through a program that used specific ports, e.g., Ports 5005-5008. Starting a week ago, I could no longer ping the Win 7 systems; nor could each Win 7 system ping itself: got...