Are your win 7 desktops pulling any
other Group policy rules? ie: desktop policies & user policies?
If it's just the mapped network drives failing, checkthe NETLOGON for the .vbs or .bat that should be mapping these drives. I've seen issues with the .vbs scripts not running correctly... The .BAT mappings definately work.
you may have to add a delete line to the bat file:
net use x: /delete /y
net use x: \\server\sharename