I took my XP laptop (running Avast Antivirus, no firewall, IE8) to a local community college and got on their network. I was getting funky security warnings on the college network, so I didn't surf, and turned off my wireless immediately. Upon coming home to my home netork, I can't print on either network printer or access other machines over the network. I get the you don't have permission error, either from my machine going to others, or others going to my machine.
I still can access the Internet and collect email, however. I've checked the firewall settings and nothing has changed.