Tabbing Between Windows Stopped Working

[anaxcrosswords]

Hope I can explain this so it's understood!
I'm on Win XP and my browser is Firefox. A few days ago I renewed my Avast! subscription (which I only mention because this problem seems to have started immediately after I did that).
If I have more than one application open, tabbing between them by clicking on the taskbar no longer works. Instead, the taskbar item just flashes - the only way to return to the app I want is to minimise anything that blocks my view and thus ability to click somewhere on the open window. In some cases, when I click the taskbar item I get the animation which represents the window I want opening, but it doesn't actually appear.
There's another symptom, too. If I close an application which sits on top of another, the title bar at the top is the 'inactive' pale blue. In the case of an onboard application this can be rectified by minimising then maximising the window. In Firefox, it's cured either by doing that or by clicking to open another tab.
Does anyone know what's happening? It's a bit annoying because when I'm working (on crosswords) I tend to have at least 4 applications open; a right pain to have to keep minimising windows instead of just tabbing or using the taskbar. I have cold booted the system at least twice, but to no effect.

[galeck1947]

There is a bug in the latest version of AVAST which is causing this . Avast are working on a fix for it but suggest you uninstall it and reinstall it. If you have access to an older version install it for now.