On how many PCs can I use Avast Free?

So far I’ve been using 4.8 on two computers, and for a while on a third. I am sure this does not violate the terms of use, but I’m wondering if a problem might occur. I have a PC and a laptop and I pretty much take care of another PC. I wouldn’t like to register several times and it is unlikely that I’ll use my registration of Avast 5 on any more PCs.

http://forum.avast.com/index.php?topic=59443.msg500957#msg500957