Thankyou Asyn.
I tried this but no effect.
Firewall still reports it is “OFF” and still get error message (see previous post) when try to switch on.
bobo1, thanks for your advice, windows did find and fix some errors, but still no luck with the Avast! firewall or with the other problem (possibly related) with XP reported in Bleeping Computer forum:- http://www.bleepingcomputer.com/forums/topic479361.html
Solved thanks to DJBone with a fresh installation of Avast! plus a helpful email from Avast Technical support. Cheers Guys! :)))