want to allow e-mail but no web browsing

Was recently asked this by somebody and wanted your advice: what’s the best/easiest/cost effective way to allow e-mail access on a LAN (25 pc’s w/Win98, 1 NT4.0 server) but disallow internet web browsing – there is no exchange server setup currently; DSL is installed from DSL modem to a router (linksys)connected to hub.

I know they can just install e-mail software and no browser application but is there any other ways? Granted, Win98 doesn’t have any real security or password level access built-in, but the company is not ready to move to another OS on workstations yet.

Any advice appreciated!

thanx!