+1 for Microsoft Security Essentials (MSE). This question is asked in this forum a lot and this tends to be the winner most times. It is available as a free download. Visit http://www.microsoft.com/Security_Essentials/ to learn more.
There are several recommended for Windows 7. I can name several that I personally have used, including Avir Avira Personal, Kingsoft, Panda, Iobit Security 360, Rising, Avast, G Data and Threatfire. Microsoft Security Essentials is my favorite one. And, if you are going to use that one, remove all traces of any other antivirus program first before installing. If you add another one to it, do not add Avast or G Data, because they will disable MSE. G Data is comprised of two engines: Avast and BitDefender. Both Panda and Kingsoft are cloud-based, and both are good for that reason. Kingsoft is smaller that Panda, and I like it for that reason.
I also tried Immunet Protect, which is cloud-based and only 1.28 MB. The jury is out on that, except for a report I found that indicates that it is not very effective in detecting or removing malware. I checked with the Better Business Bureau and there were no complaints against that company, which is located in Palo Alto, CA. This is a review of it:
No viruses were detected by any of my antivirus programs on Immunet after I tried it, but my CCleaner v3.04.1389 (64 bit) detected [Immunet] 00000000-108E688 as W32.Trojan.68f3, and quarantined it. I then uninstalled Immunet just to be safe. It might have just been a false detection, but I could not be sure.
I did try Prevx 3.0 but it would not update for me and therefore ran only a few minutes. You might wait until Prevx 4.0 is released to see if that works. I believe the company might have been out of business for a long time, because I had tried a few months ago to contact someone and no one replied. Prevx 3.0 is supposedly cloud-based, also.
I also used Vipre, mentioned in the PC Magazine article, and it works well as a standalone antivirus program, but like Avast and G Data, it disables Microsoft Security Essentials and Windows Defender, so it is not recommended to be used with either one of them.