I have even older P4 computer running XP and AVAST. And it's doing just fine.
This one is a dual core, so it is that much faster.
Yes they are fully updated SP3, with updates enabled.
And, again, Avast worked fine until that last update. And I am not even sure if it was the code update or the registration (I think - code) that did it in, but, hey - these computers are fast enough for me to keep them, I am not a gamer, I browse Internet and read email, and go on forums - this is my activity and I do not need Sandy Bridge for that.
And other antiviruses work just fine, so "Hey Avast probably shouldn't be run on older computers" is lame.
Now as for the other software, I have Spybot installed on all computers. Again, so far they have been working fine together.
And AdAware did not ask how old it is. It just works.