I have Kasppersky Internet Security installed on Win 7 64-bit Home Premium and two days ago Aero turned itself off automatically, then the PC was unresponsive for about two minutes, then it came back on, it was very weird.
What I know is that at random hours, the system becomes unstable, Flash videos work very slowly, Firefox does not render the menus in time (takes about 1 minute to render a menu)... I was starting to think something was wrong... maybe it's this.
Anyway, I just turned off Aero for now.
This goes to show that the 64-bit versions of Windows are not more secure than the 32 bit ones by default. Every version has it's own strengths and weaknesses .