i’ve been using Avast for some time and think its great my pc is windows XP home,In the past few days my pc has been locking up after about 10 minutes,
sometimes a a lot longer. I’ve narrowed it down to Avast as everything is ok when I disable it… any ideas ???
What exactly does it mean that you “disable avast”?
What if you only stop its Standard Shield provider - does it help?
I right click on the icon and stop all resources
Welcome to the forums, sandyclark. 
Instead of stopping all resources, why not try stopping each provider one at a time to see which provider might be causing a problem?
Right click the “a” ball in the Task tray > mouse over Stop Provider > select one provider at a time to stop > run with this provider stopped until the problem occurs again > restart this provider and then stop another provider > do this until the problem no longer happens.
As Igor suggested, perhaps stopping the Standard Shield first might be best. But, it would not hurt to also try stopping the other providers one at a time to observe the results.
This way, better information can be obtained as to the problem you are experiencing.