Since I've had broadband, about 5 years,I've used AVG Free and ZoneAlarm with no problems, both excellent.
I had a new computer at Christmas so decided to use Norton AV and firewall which is supplied free by BT Broadband.
From day 1 my computer kept crashing and I tried to delete Norton but it wouldn't let me so I did a system restore which finally got rid of it. I then downloaded AVG and ZoneAlarm and no more problems. My computer came with a Vista Upgrade which I did once I received the disc. AVG still downloads updates but I now have to restart for them to take affect. ZoneAlarm, last time I looked, will still not work with Vista.
Putting Vista on, although it was a free upgrade, proved very costly.
I have an Epson printer that no longer has a task monitor and prints so slowly that it stops half way through. I also have programmes that are of no use because they don't work even after running the Vista Update Advisor which told me there was only one programme that wasn't compatible.
On my new computer I have a dual graphics card but Vista can't recognise two so only updated the drivers for one of them so instead of having two 256mb cards I now only have one. Vista? if you have no need to don't go near it, at least for 12 months when hopefully things might have settled down.