Can’t you delete the correspondent avast! entry in the Registry?

Anyway, I suggest you to use Startup Delayer, which allows programs to start in a more orderly fashion by setting how many seconds after Windows has started, to load each program. The effect that this gives is that you gain control of the system much faster, and the program that was launched will launch faster as it does not have to share the CPU with other programs. You can see links for downloading it at http://www.r2.com.au/downloads/index.html

You can manual add or delete programs at startup (for all users, current user, registry and start up folders)…

I suggest you let ashDisp.exe and ashmaisv.exe running at the startup, besides the links for avast! services (which I do not remember what they are in Windows ME).