If I understand you correctly, in your network ( presume home) of 3 PCs with Win 7 beta, Win Vista and Windows Server 2003 you are able to access the administrative shares between the VISTA and Windows Server 2003 in both directions.
But I am not sure if you meant you can or cannot access either of these shares of the older Windows systems FROM Win7 PC.
I guess your problem is your inability to access Windows Admin share created in the Win 7 PC from the VISTA and Windows Server 2003 Systems.
For the last problem you could tweak the resgistry in Windows 7:
Click start ( Windows Globe icon)
Type: regedit in the "Search Programs and files" box
Press the [ENTER/Return] Key
In the left, browse to the following Hive >KEYS: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\system\
Export and Save that before editing just in case you run into problem
Right-click a blank area in the right pane of step 4