Well, as I understand it:
You install Avast, but WinXP does not recognize it and the Win Security Center gives this shield with a red x in it.
But Avast is up and running - the Avast ball is there and has no indication of malfunction. Right?
So, you may want to try a repair installation (Control Panel -> Software -> Avast (doubleclick, scroll down to "Repair", select). Then reboot.
If that does not cure it, you can disable the WinXP monitoring for AntiVirus Software in the Security Center. Avast itself will keep you informed about its status in the system tray via the a-ball.