Do you have any other security software installed, adaware, spybot S&D, etc. ?
Try a repair of avast. Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow. You need to be on-line to do this. However, I'm not sure if this option is available to win98se ?
If not, try, uninstall, reboot, install, reboot.