I don’t think it is much of an imminent issue here as avast5/6 use the same definitions, avast4.8 uses a different VPS stream and is on life support pending the switch being thrown.

However, win2k should still work with avast6, provided you have full updates to SP4 and rollup1.

Which is pretty much what you need for avast5 anyway. You also need - MS Visual C++ 2008 Redistributable. Again this is I believe required for avast5.

So you could give it a try.