The last free alternative to avast! that I used on a W98 system was Rising free which is pretty good click here . Now it doesn't claim to be W98 compatible but it didn't when I used it a year ago either so it may be worth a try.
EAV is W98 compatible click here but it's a bit of an oddball and not as far as I know tested by VirusBulletin or any of the other usual bodies.
ClamWin click here works with W98 but has no real time cover, so a real backward step from avast!
Then you're left with some new stuff that I've never tried but 'may' work with 98 Zilya click here , Preventon click here . Neither are too specific on range of OS supported.