Anti-viruses are flawed by nature in the fact that they only catch a small amount of the viruses in-the-wild. They rely on heuristics to determine if a known pattern/file-signature matches that of a computer virus. So you get quite a few false positives, and not enough alerts on zero-day viruses.
The safest thing you can do if you have an older computer (WinXP/Vista) is to upgrade to GNU/Linux. It's free and will resolve your problems pretty much for good.
However if you must continue to use Windows, don't believe the hype. Try to find one within your budget that offers decent protection, AV-Test is a good source to keep track of the winners and loosers each year:
Test antivirus software for Windows 7 - August 2014 | AV-TEST <-- click
Free ones such as AVG have been in the top 5 for years.
The trend now is moving towards cloud-scanning, which simply uploads large portions of your computer into the cloud to be scanned with multiple anti-viruses since none of them are they great. These coalitions do help catch viruses moderatly better, but aren't great on privacy.
See Also:
Antivirus is Dead: Long Live Antivirus! — Krebs on Security
And value of meta-data:
The Data Center JournalSecurity, Privacy and Metadata - The Data Center Journal
FYI: Files contain unique signatures (metadata) which are sent to the cloud servers, though I was unable to find an article on the subject, we can safely presume that unique files sent to those servers are easily traced and the IP recorded for "anti-terror" use. Say no to the cloud; unless you want to build your own private cloud. -->
Take Raspberry Pi, build your own private cloud | ZDNet