If you have used the Stop On-Access protection that is as far as you can go without using the three finger salute (Ctrl+Alt+Del) and end any avast programs completely, but that is messy and you would have to reboot to restart those elements (Safe mode is a better option, see below).
Win98's defrag is very fragile and all sorts of programs need to be shut down so it is advisable to run a defrag from safe mode avast doesn't start in safe mode, nor will other programs that might get in the way.