There have been a few people reporting some problems with win98 after an avast program update (been a while since the last one though).
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.
Or you could try the
avast! VPS Update - Manual Download, download and save the file, go off line and double click the file it should install a complete VPS file.
If that doesn't work try, uninstall, reboot, install, reboot.
It would probably be best to download the latest version of avast and save it to your HDD, somewhere you can find it again. Use that when you reinstall.