I spent the last two days trying to figure out why I could not access my Vista PC from my XP PC...I could when password protection was turned off on the Vista box but not while it was on. I could see and access everything on the XP machine but not vice versa...so after two days of reconfiguring my wireless network/router, workgroup, etc., quintuple checking my passkeys, endless reboots, etc., it turns out it was a result of me changing my username on the Vista account, to which I assume there is no fix. What a pain!

Enable guest account under control panel>user accounts and that should work