MSE (Microsoft Security Essentials) while its still free!
Because it simply does a phenomenal job versus Spyware and Viruses! Allot of people are confused that it takes a long time to scan, but to be honest with you I've noticed that very mainly the first time it takes the longest to perform a full scan until it learns your system!
I have configured it to my computer to join the Advanced Membership for free(while it lasts) and I have to say I am very impressed!
I just recently removed 2 Trojan Horse viruses that even my previous Antivirus software couldn't detect!