How much RAM do you have on this system ?
What is the CPU ?

It sounds like this could be after an update (which could be using a lot of system resources, depending on what you have) and Outpost would rightly tell you things have changed depending on your Outpost settings.

As you can see from my signature directly below this post, I use XP Pro SP3, the latest avast version and the latest version of Outpost Firewall Pro and don’t get this issue.

Can you attach a screen shot of the outpost changed avast process screen.