Most commonly, the reasons a given person can't access a share on an NT/2K/XP box are:

1. The networking feature "NetBios over TCP/IP" has been disabled on either the client or server. It's pretty far buried in the network properties.

2. The server has disabled the user account named "Guest". (This is normally disabled by default.) If the share is set with permissions like this...

UserOne: Read/Write
UserTwo: Read/Write
Everyone: Read

... then someone who isn't UserOne or UserTwo automatically connects with the account name of "Guest". So if you've disabled "Guest", then they can't access the share.