Sorry, but I have to disagree.
On my PIV 3.0 gH Ram 512 Mb WXP, Avast actually blocks the hibernate process (it comes to the "Hibernating" screen and it doesn't go any further, so that I have to crash my pc to restart working).
My computer is hibernation-enabled: indeed, when I stop all the protection servers, my computer does hibernate.
It doesn't when the scanners are active. I've had this problem since the 4 version (when I first installed avast!).