I've only ever had MS Security Essentials installed - which has worked flawlessly. My system is not complex - I have fairly up-to-date hardware running 64 bit Windows 7, and just a few programs like MS Office and iTunes installed.
Recently I installed an OCZ Synapse SSD caching drive ... still no problems.
Then, a friend recommended Bitdefender, so I bought the 2012 Total Security Suite. MS Security Essentials uninstalled fine. I re-booted. Installed Bitdefender. From the outset, on-access scanning will not turn on, and no amount of re-booting or trying to prod the settings into action would rectify the problem. OK - I uninstalled that, and installed the 2013 beta product of Bitdefender Total Security.
Now, on-access scanning works, but the anti-spam won't turn on, and the control panel shows a Bitdefender update downloading ... and seems 'stuck' on that message, without any download actually occurring, or at least not completing.
Given the foregoing, I do not see how one can have any confidence in the Bitdefender product.
I have un-installed it, and gone back to MS Security Essentials.