I've run accross situations similar to yours and they have all been firewall issues.
XP service pack 2 default installs the XP firewall as on. Zone alarm can mess it up also with its firewall. AOLcan mess it up depending on what phase the moon is in
Another thing that can mess up networks is a virus.
Try an f8 boot into safe mode and pick safe mode with network support and ping your router. If it pings ok, its a firewall issue.
ipconfig /? from command prompt will show you the reset options for your nic.
Usually /release and /renew
Don't be too quick if it pings ok--it can take a while for the computer to show up on your network--like 5 minutes






sorry for my ignorance...
