I just had the misfortune of having to repair my installation for no apparent reason. The first warning I got was Vista Security Center telling me Bitdefender was not running. Also the unusually low memory usage (just over 30MB in total for the 3 running processes) seemed too good to be true.
Bitdefender itself had a green tick. But, I checked on the settings and even though it was in Autopilot mode it was completely messed up. All rules were gone, realtime scan was disabled and it was ticked in Custom mode for antivirus. Consequently as it would not respond to my attempts to change these back to normal I had to repair the program. I have just downloaded and applied the database and program updates.
I will now monitor this carefully over the next few hours to see if the latest updates were the cause.
Computer hardware passes all health tests.
OS Vista Home Premium SP2 64-bit (AMD Athlon 64 x2 4400+ 2.2Ghz, 4GB RAM)
BDAV Plus build 17.28.0.1191
Virus signatures 11627579
Engine 7.55521