I guess you missed the bit about once on your system they can be difficult to remove, in use, protected, hidden by rootkit, etc. you system your choice.
If you haven’t got the RAM you could always upgrade it, RAM is reasonably inexpensive at the moment assuming you can find some that matches your old hardware. You don’t mention how much RAM you have or what CPU on your system.
My system is a PIII 500 with 256M of RAM, Win2K SP4.
It works not bad with AVAST full install and all the providers but I (being dumb probably) don’t get how the providers increase the system protection since they detect the same things as the Standard Shield. All the possible threats must be written, accessed or executed on my system before they become dangerous. It doesn’t look like the providers can detect something that Standard Shield misses.
So at the end I think it is not worth it loading all the providers in exchange of the gain in protection.
Lets assume that you have an undetected trijan on your system it is free to download more malware, we see examples of this in the forums where the web shield block content before it is downloaded. That same malware could also try to protect what it downloads when it arrives on your system.
Currently in the forums there is one Topic where avast reports a warning/error that it can’t scan the file because access is denied http://forum.avast.com/index.php?topic=35484.0 (are you getting the picture), so avast can’t scan it, certainly not when it is established on the system, so preventing it getting on to the system is far better than dealing with it when it is on your system.