I am running a win98se 166mhz cpu with 56mb ram yes those numbers are correct. I use to have problems completing a scan of my harddrives. I always pick thorough scan with archive scan enabled.
However I did find an odd solution useing the backup utility that comes with win98se I backup my c: drive to a folder on my d: drive than selected restore backup and selected replace all files . There was a bunch of warnings a few about avast being busy and a few about regestry settings just kept clicking ok. After the reboot the key to avast was lost likely I saved it was able to put back in. Now here is the interesting part now when avast starts it uses its safety mode skin and not that blue one. Now that it is running with safety mode skin it runs faster.
I would like to caution trying to follow what I did there is a good possibility that you might lose your entire system.