A few questions:
1) Where is the share that you are trying to access? On the Windows 2003 Server?
2) Are the new windows 7 PC's on the Domain?
3) Have you checked the Advanced sharing to se if file and print sharing is enabled on the PCs?
4) Why have you touched the Local Security Policy? By default you shouldnt need to.
5) Have you already added the Windows 2008 Server to the Domain? If so, did you make sure that the Domain and Forest was at the correct functionality?
6) Have you checked AD for errors? (dcdiag/event lofgs)
7) Check the security event logs on the Server where the Share is.
8) Disable the Anti-Virus and Firewall to test.
9) Recheck the permissions of the share. Paste a screen shot if you can.
10) Are the servers local to the users or hosted on the Web? If on another site, how are they accessed? VPN?
11) If you're redirecting paths did you change the logon scrips acordingly for Windows 7?
Just a note:
Your post was very hard to read and had a lack of structure. Its easier to diagnose if the post is easy to read.