Avira or AVG.

I even run McAfee Internet Security Suite on mine and it never causes a problem!!

Norton 360 no trouble here.X1 runs great even with aol on.

I've found Kaspersky to be fine, once it's configured the way I want it. Like all modern AV's it's more than a little bloated out of the box, but isn't too bad to configure.

I generally don't bother switching it off to use Sonar - though I have told it to ignore all processes opened by Sonar and not to bother scanning wave files on opening. And to get it's automatic updates at 6.00 a.m. when I'm unlikely to be using Sonar.

Overall it seems to make little performance difference to my desktop (quad core) whether it's on or off. It does make the PC noticably slower to boot/log on to though (to the point Vista complains about it on my Core2Duo laptop).

I can vouch for Avast! and NOD32.  I hear Kaspersky is good too.

I have heard of Norton 360 causing performance troubles, is difficult to de-install, and that it harrasses you to upgrade.  N360 is also not free.

On my DAW I use Microsoft Security Essentials, along with the Windows firewall; have been using these since January 2010 with no problems.  Both are lightweight and unobtrusive.  They update by automatically as part of Windows Update.  They're free and there's not even a yearly registration process like Avast has.

I think with the possible exception of Norton 360, there are definitely good choices.

MS WINDOWS SECURITY ESSENTIALS

free from Microsoft, the one and only. Integrates well, can be turned on and off with a single click during operation (like the metronome in most DAWs.)

I turn it on when browsing the net. During audio production its turned off.

Supercomposer


MS WINDOWS SECURITY ESSENTIALS

free from Microsoft, the one and only. Integrates well, can be turned on and off with a single click during operation (like the metronome in most DAWs.)

I turn it on when browsing the net. During audio production its turned off.

That's what I don't like about the latest version of AVG, can't turn it off.

Norton Internet Security 2011

Haven't had any performance problems when enabled.  There's an option in Norton to disable Antivirus for set period of time or until you reboot.   I disable (sometimes) along with disabling network connectivity.

Another vote for MS Security Essentials. Install it and forget it.

ms security essentials all the way here,  free and is basically part of windows.  Running it on 3 pc's and never had a single problem.