i have been using microsoft security essentials since i installed vista x64 and it is worry free. AVG is also free, but it tended to take over my machine, so MSE worked out terrifically. you will find quite a few free anti virus programs out there, just do a little research and pick the one you like best
I've used Avast also before, but for XP 64-bit, and I liked it alot. However, I use Microsoft Security Essentials (MSE) for my 7 64-bit machine and I really like it too. I think it's actually great.
I work for an IT Dept at a state univeristy and I have to deal with virus removal of all kinds at least once or twice every week. I used mostly Malwarebytes before but decided I wanted to test MSE to see how well it works. So far it's removed every virus, mainly rogue antivirus softwarebut there's been a rootkit or two, that I've tested it against the past 6 months or so and Malwarebytes doesn't even detect anything after MSE has finished. So, I started using it on my personal machine at home and it's been very good.
EDIT: Well humphreybot posted while I was typing so I didn't see his post before I submitted. But glad to see I'm not the only one!
I use Avast for Viruses and Malwarebytes for Malware. you need the one-two punch to cover both. I just run Malwarebytes manually every so often. Its also good practice to not login with your Admin for day to day surfing.
There are several suites available all of which can generally be obtained as low as $39 for a 5 pak.
Bit Defender now has 18 months for 3 PC's at $60 ...that's $13 per PC per year. Think about it.....even if you do updates only once per month, that's about $1 for being relived of that responsibility via auto updating.
Zone Alarm Internet Security is $50 for 3 PC's but if you download the free version, when it expires after 1 month, you will eventually get an e-mail offer to upgrade it for $29 or $39
Again, I used to use a variety of free ones (adaware, malwarebytes, avast, AVG and others) but with 12 PC's at home and more at the office, running around from machine to machine to do manual updates all the time was too much of a PITA.
I have used GData Internet Security in the past. It is a good dual antivirus program. Bear in mind that the Avast component will automatically disable Microsoft Security Essentials. I believe GData is a suitable MSE replacement, though.
As a long time computer tech, I have a big problem with some of these suggestions.
First, the only decent NATIVE 64-bit anti-virus I know is Nod32. The problem with Nod32 is their detection numbers fluctuate wildly, one month they may be around 98-99%, the next few months they are dropping below 90% and then back up.
The main problem with native 64-bit anti-virus solutions is that there are less than 100 known native 64bit viruses, and even then most modern security systems (NAT or dd-wrt firewalls) and anti-virus block them from being distributed anyways. Considering the number of 64 bit machines out in the wild, a 64bit virus is MUCH more rare than a virus for the Mac. Virus/malware creators go after the easiest targets aka unpatched Windows 32 bit OS systems.
Second, AVG and Malwarebytes are SERIOUS TROUBLE. They each lock down key Windows components, and many times will either prevent Windows from accessing/using them when it needs them, or even after the programs are uninstalled, they completely lock them down. Even after that the safe mode or rooted SFC (System File Checker) is not always able to replace or repair them, which means a backup of personal files and full system format and reinstall of Windows. Get both of those programs on one machine and you will see a software battle unfold as your system degenerates into something as unusable as Win3.x on a Core i5 system.
My recommendations for anything XP and newer, regardless if it is 32 or 64 bit. Each of them are free, but also have paid versions if you choose:
Avira (also known as AntiVir)
Microsoft Security Essentials (although virus detection numbers varies like Nod32's does)
Windows Defender for Vista or Win7 - There is no need for a separate spyware/malware based program until you find you are already infected, but even then 99.8% of the time Avira mentioned above blocks it.
Software firewall: Really only needed on laptops/netbooks/tablets that travel and use various networks, or home PCs that connect directly to the modem without being behind a NAT or router firewall.
Online Armor - Excellent program, but is a bit lacking when it comes to available "Advanced" options.
Comodo - I personally prefer the Comodo firewall but have also had VERY good luck and protection with Online Armor