Avast disabled/ icon missing

Forgive me for these forums are new to me…
Approx. three months ago I installed avast4.8 Home on my sisters pc(windows xp). I’ve been a user for about three years and love it(I’ve upgraded to the free 5.0-haven’t done that with her’s yet though).

Today she called me and asked me about this pop up on her son’s user file - Antivirus XP Pro. I researched it and found it’s a trojan.

Couple of questions:
a. Why didn’t Avast pick it up?

b. The Avast icon is gone on the son's profile, but Avast remains running and active on the other 
    three profiles on her computer. Also cannot access internet on her son's, but can in any other
    profile...

c. I did read the thread on here on how to remove it...when I installed Avast for her I had installed
    Anti-malwarebytes(free) also, so it is on her computer already. I can get the FixReg.exe onto a flash
    for removal (my system is Vista64, her's XP - not sure if that matters) too.

But why does this only seem to be affecting one user profile?

Why did Avast’s icon dissappear in that profile and not the others? (unless the boy disabled it - who knows)

Why wasn’t it (Antivirus XP Pro) picked up by Avast? (who knows where her kids surf–when these programs were first installed they picked up tons of stuff…the computer is running better than it was, but still needs alot of removal).

Thanx and sorry for the long post.
Any info would be appreciated

How to remove Antivirus XP Pro (Uninstall Instructions)
http://www.bleepingcomputer.com/virus-removal/remove-antivirus-xp-pro

a. Why didn't Avast pick it up?
No security program have 100% detection, if they did there would not be a virusproblem.....