What other security software do you have installed on these system ?
Whilst it shouldn't be an issue, as the differences in resource use between the 6.0 versions should be negligible, what are the specs of these systems (CPU, RAM, etc) ?
I have been using avast on an XP Pro SP3 system for many years and have never experienced this type of problem and essentially avast6 uses less resources than avast5 and avast4.8.
So I'm at a bit of a loss as to what else to suggest.