Total clean system install from scratch is always the best thing ...
... but if you have a PC with maximum of 512MB of RAM and mandatory 32MB of them is shared memory for graphics, then the system only has 480MB of RAM to use ... with Avast 4.8 absolutely no problem – but newer versions get stuck and slow down the system so much that while updating the PC is absolutely unusable; and as stated in the other thread: that update blocking can last tens of minutes.
I'll see how v9 will behave on XP with 480MB of usable RAM – and it would be nice if it would be so well programmed that it would work fine with only so few RAM; but I first have to see that before I believe it. But as today's systems have tons of RAM, I fear that "working smooth with 480MB of usable RAM" was not the main aim of the programmers for v9 ...