Is there something to do cancel automatic start with windows xp ?
1. Disable all providers, check for Yes to persist changes.
2. Remove ashDisp.exe entry of the Registry Run keys (better if you disable it through MSConfig). This is the icon on system tray.
3. Disable avast! services (Control Panel > Administrative Tools > Services)
C:\Program Files\Alwil Software\Avast4\ashServ.exe (avast! antivirus service)
C:\Program Files\Alwil Software\Avast4\ashUpdSv.exe (avast! Update Service)
C:\Program Files\Alwil Software\Avast4\ashWebSv.exe (avast! Web Scanner service)
C:\Program Files\Alwil Software\Avast4\ashMaiSv.exe (avast! e-Mail Scanner service)
4. Boot
There is no way to do it automatically or, worse, to revert automatically and start avast.
If you want two antivirus in your system (which could be dangerous), avast! should be the resident and disable the others.