What does MS have as an antivirus product?
There are two MS prodcuts. One is aimed at questionable web sites. It is named, “Windows Defender”.
The other is an MS anti-virus product named, “Microsoft Security Eseentials”. Google search or Bing search will point you to both of them.
MS Vista has “Microsoft Security Essentials”, MSE
Don’t know if it’s available for XP, 7 etc
If you don’t have any malware virus, then you just have to uninstall whatever anti virus program you have before you install the MS one. It’s free, it’s updated daily, and it really works good.
http://windows.microsoft.com/en-US/windows/security-essentials-download