A multi-application approach to security is better than a single application, whilst there some AVs like avast that has a detection rate in the high 90%, but none that give 100% protection.
The only thing is not to have more than one resident AV, but compatible anti-spyware applications like SuperAntiSpyware (SAS) and or MalwareBytes AntiMalware (MBAM) would compliment avast. These applications have free versions that provide on-demand protection only, they have paid options, but you should only have one resident anti-spyware (as mentioned for AVs).