This might seem a bit reckless, but as long as all the important data is backed up, you could try and see what happens without an AV at all.
Windows 9x doesn't really show up as a prime target anymore, for new malware; (actually, in that sense, you would probably be able to live with just the unsupported AV product).
updates to an AV add new definitions so the product can find more viruses; while I have nothing to support this, I don't see windows 9x being a big target these days; Windows Vista and 7 are the new targets that malware authors would go after, and things written for NT aren't typically something that works the same on windows 9x, especially low level things like what a virus would typically do. The operating environments are similar but are full of gotchas (to that, I can definitely attest first-hand) that require due care and attention, neither of which are usually paid with your average malware program.
Another suggestion might be to go with clamwin and use task scheduler to schedule a full scan at some time where the machine won't be in use but might be on.