hmm... I had this problem while setting up my samba server...try changing encryptpasswords to off, though i think that problem occurred later on.
also, I think the "guest ok = yes" option belongs in individual shares. Try adding "socket address = <your.ip.number.here>" (if you don't use DHCP on the linux box)
another thing to check is your windows networking settings. you should have TCP/IP bound to your LAN card, and you should have the "enable netbios over TCP/IP" box checked. you should NOT have netBEUI, IPX/SPX, or any other protocol. This is because Samba works with TCP/IP only, so you have to confugure your windoze box to do that too.