Well you will be going with something that doesn't offer the same level of protection.
There are many avast users who also use Windows Live Mail and they are surely using the Mail Shield in conjunction with WLM without and disastrous consequence having 'messed with' the WLM settings.
Avast has one of the best levels of protection against web based malware, hijacked sites and the different shields for specific functions, P2P, Network Shield, etc. etc. so the choice is entirely yours.