Ahhh... Thank you... Will that prevent the new scanner from hooking on to the data-stream as well...
It will not prevent the Network Shield to be loaded, if you mean this. As far as I know, the new mail protection doesn't monitor the network traffic as you expect. Instead, the connections to e-mail ports are redirected (by the aswTdi driver) to the e-mail scanner. So, all you prevent this way is basically just the automatic redirection (which you will set in the e-mail client instead).
If I add that parameter before upgrdaing the XP setup to avast! 4.5 will everything work as before then, or do I need to change something (port numbers or anything else)?
Honestly, I don't know - it's worth trying. If the update resets your settings (because it doesn't doesn't check for the parameter), just run the Mail Protection Wizard afterwards.
Will this "work in 98 mode" switch be there in future versions too?
I think it will. Since the necessary code is there anyway (to support Win 9x), there's no reason to remove it (besides, it may be useful on some strange configurations).