WLM does not conflict with avast, as to your question if it is worth installing well that is personal preference, i used to use WLM then changed it to thunderbird since it was a bit lighter and avast was able to scan my mail with thunderbird but now iv decided to delet thunderbird and just access my mail via hotmail.
WLM does have alot of features though if you use all of them but you can untick the componants you dont want.