Superantispyware pro and malwarebytes pro do not conflict with avast but if you only want free stuff then stick with avast free and malwarebytes free which you can update manually and run a scan with once a week, get a decent free firewall since the xp firewall is no good, privatefirewall is good and very light on the system, online armor is very strong and simple but slightly heavyer on system resources, both of these firewall’s have a hips monitoring componant.
Yes ccleaner will cleanup any tracking cookies and the new version will do a much better job of cleaning up old registry entries too.