I bought a lap top about two years ago that came with Vista preinstalled
What a nightmare! It came w/McAfee which, at the time at least, was incompatible with Vista (along with almost everything else). Problem, after problem. I had McAfee on my PC for years and already knew it was useless trying to solve issues through their tech support. Their standard answer for everything is "you need to reinstall your windows" - as though that were a simple thing. At one point my PC monitor started randomnly blacking out. I tried everything to pinpoint/fix the problem and finally had to call a computer technician. Turns out the culprit was McAfee. The software was having problems with the newest Windows update that was automatically installed on my computer. The tech said they had had several customers w/the same problem. Their company had contacted McAfee and they verified the issue but said they did not have a work around. Period. I uninstalled McAfee and the problem went away.
I spent a couple of days researching different anti virus software and in the end chose AVG. I have been using it for two years now and have never had a single problem. I only pay $49 per year for three computers and their tech support is very attentive. The reason I decided upon them was their reputation for using less resources and being less intrusive on other software. My husband's employer uses Norton. He was having so many software conflict issues that he finally uninstalled Norton and purchased AVG for his laptop. He has been very happy with it as well. I am not on a network though, so you would have to check that out.