Slow network drives after SP1 install

We're experiencing really poor network drive performance with Win7 Enterprise SP1 machines e.g. 3 laptops (all Dell D630 with same BIOS ver, built with SCCM, on same domain, in same OU, tested with same user which has local admin rights, same program features, same NIC settings, tested on same network connection, UAC turned off, and connecting to same share on one particular Server 2003 sp2 machine - other server 2003 sp2 network drives are fine).

The only difference is 1 laptop doesn't have SP1, the other 2 do. On the one without SP1, network drives open straight away - On the SP1 machines a green progress bar takes minutes to traverse the explorer bar for every folder we try to open. (this problem doesn't happen with XP, Vista, or Win 7 no SP on same server)

In summary - The problem only seems to be between Win 7 SP1 machines and one particular 2003 SP2 Server.

Any ideas welcome...