I'd switch to linux or unix right away! Windows 98 is known for some big security problems, and because it's older, it's been more hacked than newer versions of windows too. But like i said, for servers, drop windows and move to *nix!! You gain a lot by branching out and sticking out the tough parts of *nix.
Of course... a downside to your position is hardware... which makes me say, stick to windows if you know it's going to work at least
Also, i like the software you can get running on *nix better than the stuff on windows. I never did figure out Windows SMTP server...