Two antivirus is mainly not recommended.
Antivirus programs by their very nature, have to integrate themselves into the file system of the operating system (Windows in this discussion) so deeply that they can intercept every call for file operations and check if that is legitimate. This introduces some performance penalty, so figure out that the decrease of performance of your pc will be at least doubly inflicted. Also and mostly important to think about is the problem that two antivirus introduce in the risk of compatibility issues and system inestability.
So the best option is installing a good antivirus (one) and an antispyware program for complement. This combination is the most accurate option.
Actually there are some good antivirus and most of them are free. Avast! is a good option but its actualization service consumes so many resources. Avira is another antivirus that is also very popular for its performance and few use of resources. The one of microsoft is also interesting. And there are more.
About the antispyware program, as many people answer you, Malwarebytes is the really efficient.
And always remember: an antivirus does not replace common sence.