The latest worst application I tested is Kaspersky Anti-virus personal 5, my computer was freezed when scanning with KAV Personal 5 by its default settings, I seemed to can't even move my mouse from time to time while KAV tried to scan archives/packed files in C:\Windows\... folders.
I know KAV is excellent antivirus in terms of detection rates but when it comes to balancing between security and performance I think KAV is not for me.