Recommend Some Anti-Virus Softwares Compatible with Windows 7
Our Recommendation
Microsoft Security Essentials is a free anti-virus and it can work really well. Most importantly, it won't slow down your computer.

Note: Generally speaking, software that works on Windows Vista should work fine on Windows 7 as well, except a software that installs low-level system components, such as an Anti-Virus, firewalls, or similar. These packages are usually designed to run on specific versions of Windows.
Windows AntiVirus Detection
After you first install Windows 7, you'll get a popup balloon message by saying that you need to find an antivirus program online.

Once you've installed a compatible package, you will find the balloon message will go away and you'll see a message in the Action Center telling you that "Virus protection" is On:

Actually, the packages should also hook into the Windows 7 update mechanism and let you know when your virus definitions are out of date.

Note: On to the list, I took a screenshot of the Action Center screen proving that each package installs and is recognized properly by Windows 7, but I also ran through a couple of quick tests for each package.
AVG Anti-Virus Free Edition
AVG Free is one of the most popular anti-virus softwares now. Fortunately, it is not only work perfectly under Windows 7, but it's on the list of a small number of software packages that are officially compatible per Microsoft.
You'll notice that it also offers Anti-Spyware protection, and Windows recommends that you only run a single Anti-Spyware application:

Avira AntiVir Personal Edition
Avira is a popular freeware antivirus with a number of the regulars in favor of it.

Note: This software works without any issues that I could find during my quick testing.
Norton AntiVirus 2009
There is no doubt Norton AntiVirus 2009 works just fine. They are also on Microsoft's list of compatible software for Windows 7.

Note: Since it also provides anti-spyware protection, maybe you would like to disable Windows Defender for performance reasons.
Avast! AntiVirus Home
Also this one works just fine. You'll find that this package provides Anti-Spyware protection as well.

Microsoft Windows Live OneCare:
What surprise me is that this one doesn't even install under Windows 7, just giving you a big fat error message (I guess Microsoft still has some coordination problems.).

In fact, OneCare is going to be discontinued in June 2009, with Microsoft switching to offering a free product instead.
Kapersky Anti-Virus 2009
I list it here not only it works well but also it is preferred by a number of computer users.

McAfee VirusScan:
Actually it is reported to be not working by multiple readers.
Also Working Per Reader Comments
These packages are reported to be working by the readers in the comments below:
-PCGuard Anti-Virus from Virgina Media Broadband
-ESET Nod32 Anti-Virus
-Microsoft Forefront Client Security (Enterprise)
Important Note
Just because all of these packages install and run doesn't mean they are necessarily "supported" by the vendors.
If you are looking for an Anti-Virus package, this list can help you much.
Official Microsoft Anti-Virus Partners Page at microsoft.com
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