Hi everyone!
I updated today to Avast Home 4.6.603 but in Windows XP SP2 Security Center the program version appears as 4.5.603.
I guess it must be a very small bug/incompatibility.

Yes. :slight_smile:
Please see e.g.
http://forum.avast.com/index.php?topic=11300.0 or
http://forum.avast.com/index.php?topic=11420.0