There is no getting away from the fact that all the AV packages place a
significant load on the machines these days - especially when they scan
for non virus malware as well.

That's quite true, but in the meantime, i'd suggest ditching AVG
(whose detection rate has always seemed poor in real-world experience
to Avast), and do a general sweep of the system to make sure there's
nothing untoward running on it.

For the OP...

First things first, press CTRL-SHIFT-ESC and see how many processes
she has running - a "clean" system (new install) *with* Avast and
ZoneAlarm generally has 33-34.

My heavily loaded system (over 18 months old, and has a _lot_ of apps
installed) currently has 43 processes, but of these, two are my
newsreader (one for the main app, one for this message), an email
program, and a browser. Another task is the task manager which I still
have open as I type now, so call it 42 :-p

Also, check the amount of memory in the machine - you want to look at
the "commit charge" in the status bar of the task manager. Make sure
she has at least as much physical memory in the machine as she is for
the "commit charge" or it'll just page to disk permanently.

Has it been defragged lately ? - if not, I can recommend JKDefrag
(google for it) - it's small, system friendly, and very efficient.
There's no "interface" for it as such, you can simply close the window
to stop it running (it'll finish the file it's working on first)

If she has significantly more processes, it's worth going to
hijackthis.de and downloading HijackThis - run it with the "save to
logfile" option, and copy and paste the results to the window on the
same site to give yourself an idea what each thing might be as the
machine boots. Take the results with a pinch of salt though, as it
doesn't know every variation of every application in the world :-}