This is an interesting one that i think i have racked my brain to death over. I have a multi-purpose multi OS network running in my house. Currently it is a windows 7 beta Computer, a windows XP machine which is my main computer, a FreeNAS server for file storage, a Ubuntu 9.04 machine working as a MythTV box, and 2 macbook laptops.
Everything was running fine until this week, now suddenly the windows XP machine does not see anything on the network. it has internet just fine, and works with everything i have thrown at it (p2p, online gaming, streaming video) but it cannot see the other machines on the network, which it has been doing fine all along. Also, when on one of the macbook pro's they see each other and the freenas, the windows 7 and the ubuntu box just fine, but they do not see the windows XP machine. It appears that the windows XP machine has isolated itself on the network for some odd reason.
I have tried different NIC cards and re-installing the current NIC. i also hard set the IP for the computer into the router thinking maybe that would help, but so far nothing has worked. The firewall is turned off on the machine and i can't figure out what has changed in the past week for this to happen. No new software has been installed as far as i know and i have not been using it much except to print out some things from the web.