As you all know Anti-Virus software and other diagnostic software can slow down the PC massively, it is unreal how much it chokes the PC up, what would your alternative be for a customer.

I personally don't use it as if your Windows is updated and you don't download and install any dodgy files then you can't get infected, simple. All the RPC worm entry points are patched etc...

So should i just go for the Windows update route and forget AV?

I could install a 3rd party firewall but then that requires you to allow, or disallow, trusted connections such as IE when you connect out, yep just clicking yes or no, now this is easy to you and me but to a novice who's PC you are fixing, well it is an added struggle for them...

Err, I wouldn't ever advise not having any sort of protection - you never know what's on a cd/usb stick etc. (even MS once distributed a virus on some cd's of Office 97).

AVG is not particularly instrusive so I'm surprised to see you're noticing problems.

Persoannly I'd go for the ESET Home Sec suite (firewall, av etc.) which has a particularly small "footprint".

See : www.eset.co.uk

Def avoid Norton !

As already said, AVG doesn't really hog to much in the way of resources. If it is causing a problem, have you tried unistalling and installing the latest file from their site?

system specs of the computer?

I ALWAYS put on a 3rd party firewall and Antivirus.

My current fave is comodo.

http://www.comodo.com/home/internet-security/antivirus.php

I would uninstall/ reinstall and see if that helps the problem.
If not.. another vote for ESET's Smart Security :)

I am happy with BitDefender Antivirus 2009 (although I did like 2008 better)