Some people have asked me what I recommend for an AV... I'm currently using NOD32 v4 from ESET. I have been increasingly impressed at their extremely efficient yet effective realtime scanner. However due to their abandonment of OEM channels I'm taking a serious look at GDATA as being an AV to sell to my customers.Products such as BitDefender which have attempted very thorough realtime protection fail in my opinion on I/O efficiencies and definitely impact the computer significantly in terms of performance. So as another person commented, not all realtime scanners are created equal and some are very system intensive.I/O is the primary factor of system performance in realtime scanners with a secondary factor being CPU usage. If you are curious about how much I/O your current AV is consuming, fire up taskmanger > processes tab > view menu > select columns > check the boxes of: I/O Read Bytes, I/O Write Bytes, and I/O Other Bytes. Monitor those values for a while in comparison to other processes on your computer and you will soon see the affect of your AV! If you want to see more realtime data (I/O delta) download Process Explorer from sysinternals.com (although I think the taskmanager in Vista/Win7 has a delta I/O option)