Share files - Ubuntu 12.4 and Windows 7 - one network - password not accepted

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http://askubuntu.com – I ask this question in SuperUser but no one helps me. I hope to get more attention here. I have three computers connected in one network by modem. I want to share files in this network in the most easy way (I have read about solutions using Samba). So, I have three machines: One with Windows 7 One with Windows XP One with Ubuntu 12.04 and I have the following situation: The windows PCs can share files between each other. The windows PCs can see that Ubuntu's one is in the network The PC with Ubuntu can see only the PC with Windows 7, but when I click on a folder it ask to enter the net (HowTos)