I'm using Kaspersky Pure 2, all I can say is its the best antivirus suite you can ever get. I've tried almost all antivirus, including Bitdefender, all are either causes lag, bad detection rates, start up slowdowns, slow when opening files, firewall problems.
The key points to choose Kaspersky are: (In my opinion)
-One of the top tier protection you can get
-Doesn't cause lag, sluggishness or delay when browsing the internet or opening files.
-No firewall problems (It gets really annoying when you want to game with an antivirus that's keep blocking each applications or connections)
-COD NAT always open
(Unlike other antivirus, its either Moderate or Strict)
However there's a few downsides with Kaspersky:
-Slow scanning (Could be hard drive too large)
-High RAM usage (Around 300MB when idle, using Windows 8 x64 with 6GB Memory, so there's no problem for me
)
Conclusion: Good for install and forget
(Note: I don't think Bitdefender is a install and forget product since it lags the internet heavily)