Check with cable operators to see if they support this system. There is a free solution that could turn an old 486 into a perfectly serviceable firewall, but it's not for the faint of heart. You'll have to have an understanding of networking basics. It's a skinny version of Linux, a free operating system that tastes a lot like Unix. This version is so skinny that your 486 machine doesn't need a hard disk because it will run from a floppy.
Cable modems are an exceptionally good value for the money.
With a little caution and preparation, your would-be machine cracker will move on to softer targets. All rights reserved.