It's getting risky to run Windows 98 on Internet-connected computers as security updates are no longer available for that OS (which is, after all, over ten years old now).
That having been said, I use and recommend "avast!" which has a free "Home Edition" available. It supports Windows versions back to Windows 95, and it's done very well in magazine comparisons against the expensive ones.
http://www.avast.com/eng/home_edition.html
