I was trying to map a network drive on PC A to PC B on a small network...with poor results.  The Map Network Drive wizard kept complaining that it couldn't find the share I had set up on B...even though I'd used that share successfully dozens of times before with other machines. 

I made sure that both machines were in the same workgroup, but that didn't do it.  I finally disabled the Window XP Internet Connection Firewall on PC A and that did the trick.  I thought that firewall was supposed to prevent traffic in only one direction (in), but apparently it prevents network activity in the out direction too.