I use Microsoft Security Essentials (Free) as well as Kaspersky Internet Security (Paid). I use 2 programmes, you know the saying better safe than sorry so if one misses something the other should detect it (Hopefully). You couls always use 2 free anti virus programmes though or even just 1 if your less paranoid than me :cryvaultboy:
You shouldn't use two antivirus (or firewalls for that matter) at the same time as they may (probably will) conflict with each other.
Personally I use MSE, Zone Alarm's free firewall, Malwarebyte's Anti Malware, Virus Effect Remover and Spybot Search and Destroy. All free, none conflict with each other, all great imo :thumbsup: