If for some reason you cannot sort it out with the far superior solutions using the Cisco as the Default Gateway and Registry/routes on clients are the only way, you could do it via Startup or Shutdown scripts wich run as a SYSTEM account (administrative privilges). Do not know enough about Pre-XP / server 2000/2003 environments to know how the group policy's work, but in 2000/2003 you can set a startup/shutdown script(s) via group policy.

Make sure that if it points toward a script on a server, that the share and NTFS permissions have read access for 'Domain Computers'.

Luke