We'd need some specifics (OS, network numbers, etc) but generally the way you do it is to set the Internet (cable) connection as your Default Gateway, then add a static route to the campus network.
i.e., "Route Add 10.0.0.0 mask 255.0.0.0 10.0.0.254"
The "10.0.0.0" network would represent your campus network, the "10.0.0.254" would be your gateway to the campus network.
If you're using NT, WIN2K, or XP you can make it "route add -p ..." to make it a persistant route (will be there after a re-boot).
With this setup, everything addresses to the campus would go to the campus gateway, everything else would go to the Internet (cable).
Good Luck
Scott