we have the usual "cannot find tree" login-problems with some of our windows 7 + nc2sp2 clients.

These problems only seem to occur when the clients are attached to Cisco Switches.
Reboot + Instant Login at the GUI leads to "cannot find tree".

We read all the stuff about turning on portfast on the cisco switches but it does not help. Is there any other way to force the client to wait until "the switch is ready".
Anyone any ideas?