PC World did a pretty in depth review in their August issue and found Avira to be the best. My Windows 7 Panda beta ran out about then so on their recommendation I am using it and it seems great so far.
I dual boot Windows 7 Ultimate x64 and XP Professional 32 and have Kaspersky on my XP. Avira seems less intrusive so far, although Kaspersky ( antivirus not internet security ) always rates extremely high it can be a pain sometimes.
I've used avira for over a year now on my three computers along with about 10 I've built with no issues so far. You do get at least one avira pop up a day but I don't mind becuse it's free and works. Six of those computers run windows 7 64 bit.
Not Free....but if you want a fine low cost alternative , can get a home site license for F-Prot for $29.95 and it covers 5 PC's. My professional society subscribes to a corporate license and they pay about $2 or $3 a seat....with technical support.
VIPRE Antivirus is another good alternative if you're into going for paid AV solutions. 1 year subscription for $49.95 for an unlimited home site license that, of course, covers every PC you personally own. I had it installed on all 7 of my systems at one point.
I initially bought a 1 year site license, extended by another full year for free through their VIPRE Points program, and I just bought another 3 year site license as I was extremely satisfied with the program.
I recommend Avast! I use it in XP and Vista and my bro uses it in Win7. I have had no viruses since I have been using it (4-5 years) and any warnings I get are dealt with easily. Sign up for a free 1 year license and when that's up apply for another FREE licence.
Also I was using Norton before this and after installing Avast it found loads of stuff and fixed them. I will never use Norton again, neither should anyone else.
As a free AV Avira seems to be the best, as many tech sites reports.I installed this,by request,on many costumers and friends machine and they are quite satisfied.I personally prefer Kaspersky KIS or G-Data I.S. for 30 Euro on 3Pc-s per year.Is just 1 month of WaW fee for a better security and is all inclusive.Anyway like someone say earlier:It's a matter of personal opinion and taste! And depends allot of what kind of internet browsing u do,and computer use u have (torrents,warez,program cracks,etc)
Any time I fix a friend's computer now, I install MSE and the Comodo firewall. Both are clear in their various presentations to the user (virus notifications and web access requests respectively) but they have also both found almost the perfect balance between being too intrusive and annoying, and the user not even knowing the program is running. Many other AV and firewall programs are too close to either edge of that scale.