Obrigado a todos pela sua ajuda.
Isso resolveu claramente o problema.
Por segurança, citarei abaixo:
Find solution for xp lost network share from windows 7 afters couple of hours
PROBLEM :
Network shares created on Windows 7 and being accessed by Windows XP workstation are lost after a certain amout of time. XP never be able to access Share made on 7 until 7 is rebooted. You can still ping 7 from XP.
WHAT I FOUND :
Default configuration of Windows 7 (and Vista) for Network sharing is not well balanced. This must be fixed in registry. I found in event viewer (Administrative Event) a lot of SRV error #2017. This error is directly related with lost network sharing (Xp --> 7).
HOW TO FIX IT :
Start RegEdit, go here an set the following registry key to "1" : HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\LargeSystemCache
Set the following registry key to "3" : HKLM\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters\Size
Reboot Windows 7. You will never have Network share drop again. I think this fix can also be applied for Vista.
Reference : <http://alan.lamielle.net/2009/09/03/windows-7-nonpaged-pool-srv-error-2017>Windows 7 Nonpaged Pool Srv Error 2017 | alan.lamielle.net