I tried avast lately and really like is, one thing in noticed is that under windows 7 opening windows and applications seems to have slowed down considerably, were talking 3-5 seconds longer.
I installed only the on-demand scanner, no shields at all, tried disabling the graphics option and icon animation which helped a tiny bit but the deference when restoring to a pre avast image is noticeable.
Windows 7 64bit 4GB ram.