There are many free legitimate anti-virus programs that one can download from the Internet, including Kaspersky, Avira, AVG, Avast, etc. My choice for a free workable anti-virus solution is Avast. There are several reasons. Avast is easy to install. The main program interface seems simple and easy to use. Daily virus definition updates are completed with an audio confirmation that I like to hear. The free edition also seems to work well in detecting viruses and even in running pre-Windows-boot scans if necessary.
The fee-for-service versions claim to do a better job in detecting viruses, but I have found the free versions to be sufficient. Also, the former usually come with extra services not directly related to an anti-virus program, such as a built-in backup solution, clean-up tools, and performance enhancers. While these are nice, they can be utilized by other means with Windows' built-in applications, or will not actually gain you any noticeable performance power.
One caveat, however: one should always have a backup solution in place, and in functioning order, just in case your system is infected with a virus that cannot be eradicated; or just in case a system restore will not produce the desire results or not work at all. This should be so, whether you have a free anti-virus program or one for which you paid.
