Do you know of a firewall that is suitable for Windows 98?

There may be some older firewalls which are still compatible with Windows 98, but given that this version of Windows has been superseded four times (and is now over 10 years old) I would strongly suggest that you update to a more recent version of Windows. In particular, Windows 98 is not longer supported by Microsoft and no updates or security patches are being released for this product. Therefore, this has significant security implications, even if you are running a firewall. As such, for the best protection you should upgrade to at least Windows XP, if not the latest version of Windows being Windows 7. I imagine that your computer is fairly old, so you may also wish to upgrade your computer. There are very good deals to be had at the moment, with new Windows 7 computers costing less than $1000 from companies such as Dell (www.dell.com.au). I would encourage you to investigate these offerings, as the computers will perform much better and will be more secure through running a more recent operating system.