There are some functions that just aren't available in avast with win98, this is due to the age of win98 (7 year old OS and no longer being developed).
The ability to schedule a boot-time scan from within avast (with winXP) is a major feature, not to mention the ability to have transparent scanning of Internet Mail provider and Web Shield without having to manually set the proxy, etc.
All of this is down to the increased functionality of winXP and avast taking advantage of this, not any bias in favour of winXP by avast.