Samba share on Mountain Lion freezes
Considerations
For the partition, in its entirety, at the server you grant read and write privileges to everyone:

may be comparable to guest access, which does not require authentication.

Consider the following possibilities:

some communication, or attempted communication, by Windows 7 may be anonymous, without authentication (whether/how such communications would be logged by the server in a non-Server build of the OS, I don't know)
writes, or attempted writes, by Windows 7 are inappropriate for something at/around the root of the partition.

Suggestions
Increase the verbosity of logging for SMB
This may be easier to achieve with a Server build of the OS.
Diagnosis at the server when the client perceives a problem
Enable the stackshot daemon then use the key chord for sysdiagnose
Preparation: enable the daemon
sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.stackshot.plist

When the problem occurs: use the key chord for sysdiagnose.
For at least ten seconds after the chord, touch nothing.
After Finder brings to front the result of sysdiagnose: decompress the archive, consider the files that comprise the diagnosis.
References
stackshot(1) OS X Manual Page
sysdiagnose(1) OS X Manual Page