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#21
I'm not sure that using a static IP will make a difference here. It's usually best to use the DHCP reservation list for static IP's in the routers settings rather than messing with IPv4.
It may help to turn off UAC on the Windows 7 machine as this is known to conflict with XP and will cause this type of problem where they can't access each other.
Also using the same user name and password on the XP and Windows 7 machine can make this work. The following link may also be helpful. :)
SMB File Sharing Accessing windows 7 shares from XP and other devices